Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity (Share-Based Compensation) (Details)

v3.8.0.1
Stockholders' Equity (Share-Based Compensation) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 17.5 $ 11.0 $ 10.9
Unrecognized share-based compensation expense $ 11.3    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average period for recognition of unvested shares 1 year 7 months 23 days    
Restricted Stock | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Stock Option Plan | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value (USD per share) $ 23.24 $ 17.87 $ 15.06
Fair value of awards vesting $ 29.9 $ 13.5 $ 17.0
Class A-1 Common Stock | Stock Option Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 15,700,000.0    
Number of shares available for grant 1,200,000