Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation - Summary of Restricted Stock Award and Unit Activity (Details)

v3.8.0.1
Stock-Based Compensation - Summary of Restricted Stock Award and Unit Activity (Details) - Restricted stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Number of unvested restricted shares, beginning of period (shares) 613,056 484,239 387,245
Granted, number of unvested restricted shares (shares) 876,846 451,241 352,605
Vested, number of unvested restricted shares (shares) (423,977) (252,566) (236,812)
Forfeited, number of unvested restricted shares (shares) (89,898) (69,858) (18,799)
Number of unvested restricted shares, end of period (shares) 976,027 613,056 484,239
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Unvested restricted shares, weighted average grant date fair value, beginning of period (usd per share) $ 32.90 $ 43.51 $ 55.87
Granted, weighted average grant date fair value (usd per share) 15.14 27.78 35.99
Vested, weighted average grant date fair value (usd per share) 29.90 43.94 52.39
Forfeited, weighted average grant date fair value (usd per share) 27.91 33.43 45.21
Unvested restricted shares, weighted average grant date fair value,, end of period (usd per share) $ 18.71 $ 32.90 $ 43.51