Employee Stock Plans - Stock Option Valuation Assumptions (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Weighted Average Volatility Rate	[1]	26.00%	31.00%	28.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	[2]	5 years 1 month 6 days	5 years 2 months 18 days	4 years 9 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	[3]	1.90%	1.30%	1.50%

[1] Volatility is measured using historical daily price changes of the company's common stock over the expected term of the option.

[2] The expected term represents the weighted-average period the option is expected to be outstanding and is based primarily on the historical exercise behavior of employees.

[3] The risk-free interest rate is based on the U.S. Treasury zero-coupon yield with a maturity that approximates the expected term of the option.