Annual report pursuant to Section 13 and 15(d)

Share-Based Compensation (Phantom Stock) (Details)

v3.8.0.1
Share-Based Compensation (Phantom Stock) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation costs $ (22.7) $ (16.2) $ 2.5
Phantom Share Units (PSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares of phantom stock available for awards 1,000,000    
Phantom shares vested 360,481 333,570  
Total liability for share awards $ 56,584,000 $ 38,964,000  
Director Phantom Stock Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Share-based Liabilities Paid 474,000 278,000 0
Compensation costs 2,588,000 2,098,000 (916,000)
Total liability for share awards 8,436,000 5,848,000  
Employee Phantom Stock Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Share-based Liabilities Paid 3,066,000 2,442,000 1,682,000
Compensation costs 16,910,000 12,694,000 $ (1,612,000)
Total liability for share awards 48,148,000 $ 33,116,000  
Unrecognized compensation cost $ 12,700,000