Quarterly report pursuant to Section 13 or 15(d)

Variable Interest Entities (Details)

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Variable Interest Entities (Details) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Dec. 11, 2012
Oct. 03, 2012
Aug. 30, 2011
Mar. 31, 2015
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Variable Interest Entity [Line Items]                  
Impairment of equity method investment         $ 0 $ 0 $ 12,593,000 $ 0  
Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Project fund             $ 59,300,000    
Tax credit percentage             39.00%    
Percentage of recapture             100.00%    
Recapture period             7 years    
Assets         140,900,000   $ 140,900,000   $ 140,900,000
Liabilities         $ 104,200,000   $ 104,200,000   $ 104,200,000
NMTC 1 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Project fund     $ 58,300,000            
Interest rate percentage     1.00%            
NMTC 1 [Member] | Minimum [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Interest rate percentage     1.00%            
NMTC 1 [Member] | Maximum [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Interest rate percentage     3.96%            
NMTC 2 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Project fund   $ 37,700,000              
Interest rate percentage   1.00%              
NMTC 2 [Member] | Minimum [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Interest rate percentage   0.7099%              
NMTC 2 [Member] | Maximum [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Interest rate percentage   0.7693%              
NMTC 3 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Project fund $ 8,200,000                
Interest rate percentage 1.00%                
NMTC 3 [Member] | Maximum [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Interest rate percentage 1.35%                
US Bancorp [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity     $ 22,400,000            
US Bancorp [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity   $ 17,500,000              
US Bancorp [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity $ 3,800,000                
Community Development Entities [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity     $ 76,800,000            
Community Development Entities [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity   $ 55,200,000              
Community Development Entities [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Amount to other entity $ 12,000,000                
Next Generation Carrier-Class Communications Services Firm [Member]                  
Variable Interest Entity [Line Items]                  
Ownership percentage         40.80%   40.80%    
Fair value of equity method investment         $ 0   $ 0    
Impairment of equity method investment         12,600,000   12,600,000    
Next Generation Carrier-Class Communications Services Firm [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]                  
Variable Interest Entity [Line Items]                  
Project fund       $ 3,000,000          
Other Current Assets [Member] | Next Generation Carrier-Class Communications Services Firm [Member]                  
Variable Interest Entity [Line Items]                  
Notes receivable         $ 3,000,000   $ 3,000,000