Accounting for Stock-based Compensation - Cash-settled Restricted Stock Unit Activity (Details) - Cash Settled RSUs [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Non-vested Beginning Balance	463,022	446,663	537,119
Number of Shares, Granted	174,419	233,790	121,015
Number of Shares, Vested	(161,576)	(146,619)	(78,033)
Number of Shares, Cancelled	(83,949)	(70,812)	(133,438)
Number of Shares, Non-vested Ending Balance	391,916	463,022	446,663
Number of Shares, expected to vest in the future	391,916
Weighted-Average Grant Date Fair Value, Non-vested Beginning Balance	$ 35.96	$ 37.18	$ 36.36
Weighted-Average Grant Date Fair Value, Granted	42.06	34.29	39.35
Weighted-Average Grant Date Fair Value, Vested	40.78	34.70	33.29
Weighted-Average Grant Date Fair Value, Cancelled	36.43	37.55	38.14
Weighted-Average Grant Date Fair Value, Non-vested Ending Balance	38.80	$ 35.96	$ 37.18
Weighted-Average Grant Date Fair Value, expected to vest in the future	$ 38.80
Aggregate Intrinsic Value, Non-vested	$ 20,576,000
Aggregate Intrinsic Value, expected to vest in the future	$ 20,576,000