Stock-Based Compensation - Fair Value Of Stock Options Granted (Details) - Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Black-Scholes option valuation model, assumptions
Risk-free interest rate, low end of range (as a percent)	1.34%	1.06%	1.40%
Risk-free interest rate, high end of range (as a percent)	2.29%	2.27%	2.19%
Volatility factor, low end of range (as a percent)	22.25%	26.12%	26.42%
Volatility factor, high end of range (as a percent)	26.15%	33.37%	36.22%
Maximum
Black-Scholes option valuation model, assumptions
Expected term of options	5 years 6 months	10 years	7 years 9 months 18 days
Weighted-average fair value of options granted (in dollars per share)	$ 16.69	$ 26.86	$ 29.73
Minimum
Black-Scholes option valuation model, assumptions
Expected term of options	3 years 9 months 18 days	5 years 4 months 24 days	5 years 6 months
Weighted-average fair value of options granted (in dollars per share)	$ 11.95	$ 12.45	$ 16.14