Annual report pursuant to Section 13 and 15(d)

Income Taxes (Narrative) (Details)

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Income Taxes (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Income tax benefit resulting from change in tax law $ 41,626,000 $ 0 $ 0
Unrecognized tax benefits 0 0 0
Income tax interest expense 0 0 0
Income tax penalties expense 0 0 0
Employee's excess tax benefit for stock based compensation 3,397,000 0 $ 0
Operating Loss Carryforwards [Line Items]      
Cumulative effect of ASU 2016-09 adoption   7,095,000  
Retained Earnings (Deficit)      
Operating Loss Carryforwards [Line Items]      
Cumulative effect of ASU 2016-09 adoption   $ 7,077,000  
Federal      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards $ 371,168,000