Employee Stock Plans - Stock Option Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	37.00%	[1]	41.00%	[1]	39.00%	[1]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	5 years 3 months 18 days	[2]	5 years 4 months 24 days	[2]	5 years 3 months 18 days	[2]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.60%	[3]	1.00%	[3]	1.00%	[3]

[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.