Stock-Based Compensation - Fair Value Of Stock Options Granted (Details) - Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Black-Scholes option valuation model, assumptions
Risk-free interest rate, low end of range (as a percent)	2.09%	1.34%	1.06%
Risk-free interest rate, high end of range (as a percent)	2.98%	2.29%	2.27%
Volatility factor, low end of range (as a percent)	23.33%	22.25%	26.12%
Volatility factor, high end of range (as a percent)	30.22%	26.15%	33.37%
Maximum
Black-Scholes option valuation model, assumptions
Expected term of options	5 years 6 months	5 years 6 months	10 years
Weighted-average fair value of options granted (in dollars per share)	$ 12.53	$ 16.69	$ 26.86
Minimum
Black-Scholes option valuation model, assumptions
Expected term of options	2 years 9 months 18 days	3 years 9 months 18 days	5 years 4 months 24 days
Weighted-average fair value of options granted (in dollars per share)	$ 7.10	$ 11.95	$ 12.45