Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation - Performance-Based Awards (Details)

v3.8.0.1
Stock-Based Compensation - Performance-Based Awards (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
PCUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Nonvested weighted average nominal value per share (in dollars per share) $ 100.00    
Percentage of number of shares earned related to performance maximum 200.00%    
Performance period 3 years    
Maximum percent of units granted at maximum achievement level 250.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, Beginning Balance (in shares) 26,352    
Granted (in shares) 14,153 17,091 17,091
Vested (in shares) (11,022)    
Forfeited (in shares) (6,462)    
Nonvested, Ending Balance (in shares) 23,021 26,352  
Allocated share-based compensation expense $ 1.0 $ 2.3 $ 2.3
Performance-Based Phantom Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, Beginning Balance (in shares) 1,315,228    
Granted (in shares) 660,370 1,164,880 541,127
Vested (in shares) (348,962)    
Forfeited (in shares) (315,579)    
Nonvested, Ending Balance (in shares) 1,311,057 1,315,228  
Allocated share-based compensation expense $ (0.9) $ 1.3 $ 0.4