Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Stock-Based Compensation (Summary of Restricted Stock Unit Activity) (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock units outstanding, beginning, number of shares 466,475 553,289 658,760
Granted, number of shares 270,169 205,734 207,040
Vested, number of shares (176,654) (235,059) (243,765)
Forfeited, number of shares (110,515) (57,489) (68,746)
Restricted stock units outstanding, ending, number of shares 449,475 466,475 553,289
Restricted stock units outstanding, beginning, weighted average grate date fair value per share $ 47.23 $ 38.85 $ 31.03
Granted, weighted average grant date fair value per share 52.45 55.93 51.07
Vested, weighted average grant date fair value 42.32 33.98 28.09
Forfeited, weighted average grant date fair value 48.13 46.35 34.57
Restricted stock units outstanding, ending, weighted average grate date fair value per share $ 52.07 $ 47.23 $ 38.85