Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivative Instrument (Significant Assumptions and Inputs on Stock Appreciation Right Liability) (Details)

v3.8.0.1
Fair Value Measurements and Derivative Instrument (Significant Assumptions and Inputs on Stock Appreciation Right Liability) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Volatility (as a percent)   37.50%
Risk-free interest rate   2.10%
Stock Appreciation Rights (SARs)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Stock Price (USD per share) $ 39.02 $ 19.45
Stock Appreciation Rights (SARs) | Minimum    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Contractual term (in years) 1 year 1 month 2 years 1 month
Volatility (as a percent) 25.00%  
Risk-free interest rate 1.30%  
Stock Appreciation Rights (SARs) | Maximum    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Contractual term (in years) 5 years 1 month 6 years 1 month
Volatility (as a percent) 37.50%  
Risk-free interest rate 2.20%