Accounting for Stock-based Compensation - Cash-settled Restricted Stock Unit Activity (Details) - Cash Settled RSUs - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Non-vested Beginning Balance	337,406	391,916	463,022
Number of Shares, Granted	103,606	147,103	174,419
Number of Shares, Vested	(123,395)	(147,759)	(161,576)
Number of Shares, Cancelled	(21,496)	(53,854)	(83,949)
Number of Shares, Non-vested Ending Balance	296,121	337,406	391,916
Number of Shares, expected to vest in the future	276,247
Weighted-Average Grant Date Fair Value, Non-vested Beginning Balance	$ 47.73	$ 38.80	$ 35.96
Weighted-Average Grant Date Fair Value, Granted	77.03	60.84	42.06
Weighted-Average Grant Date Fair Value, Vested	44.61	38.71	40.78
Weighted-Average Grant Date Fair Value, Cancelled	53.92	43.07	36.43
Weighted-Average Grant Date Fair Value, Non-vested Ending Balance	58.83	$ 47.73	$ 38.80
Weighted-Average Grant Date Fair Value, expected to vest in the future	$ 57.98
Aggregate Intrinsic Value, Non-vested	$ 27,130,606,000
Aggregate Intrinsic Value, expected to vest in the future	$ 25,309,750,000