Stock-Based Compensation (Summary of Activity of Options Outstanding not yet Vested) (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation [Abstract]
Number of Shares, Unvested as of December 31, 2017	2,860
Number of Shares, Granted	941	1,171	1,357
Number of Shares, Vesting during period	(1,104)
Number of Shares, Forfeited/cancelled	(43)
Number of Shares, Unvested as of December 31, 2018	2,654	2,860
Weighted-Average Fair Value Per Share, Unvested as of December 31, 2017	$ 22.08
Weighted-Average Fair Value Per Share, Granted	33.01	$ 23.88	$ 19.19
Weighted-Average Fair Value Per Share, Vesting during period	21.74
Weighted-Average Fair Value Per Share, Forfeited/canceled	26.32
Weighted-Average Fair Value Per Share, Unvested as of December 31, 2018	$ 26.05	$ 22.08