Stock-based Compensation (Summary of Stock Option Activity) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, beginning of period, Shares	1,062,177	1,396,152
Granted, Shares	291,390	417,690
Exercised, Shares	(84,353)	(148,529)
Forfeited or cancelled, Shares	(50,491)	(2,334)
Outstanding, end of period, Shares	1,218,723	1,662,979
Outstanding, beginning of period, Weighted Average Exercise Price	$ 75.53	$ 68.63
Granted, Weighted Average Exercise Price	110.58	91.18
Exercised, Weighted Average Exercise Price	70.31	59.71
Forfeited or cancelled, Weighted Average Exercise Price	88.48	67.62
Outstanding, end of period, Weighted Average Exercise Price	$ 83.73	$ 75.10