Stock-Based Compensation (Stock Option Activity) (Details) - Stock Options $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Shares
Options, outstanding, beginning of period (in shares) | shares	1,439,708
Options, granted (in shares) | shares	132,832
Options, exercised (in shares) | shares	(116,590)
Options, outstanding, end of period (in shares) | shares	1,455,950
Options, vested and expected to vest (in shares) | shares	1,311,896
Options, exercisable (in shares) | shares	682,494
Weighted Average Exercise Price
Weighted average exercise price, beginning of period (per share) | $ / shares	$ 63.21
Weighted average exercise price, granted (per share) | $ / shares	98.97
Weighted average exercise price, exercised (per share) | $ / shares	58.05
Weighted average exercise price, end of period (per share) | $ / shares	66.88
Weighted average exercise price, vested and expected to vest (per share) | $ / shares	67.76
Weighted average exercise price, exercisable (per share) | $ / shares	$ 75.92
Weighted average remaining contractual term (in years)	6 years 9 months 12 days
Weighted average remaining contractual term, vested and expected to vest (in years)	6 years 6 months 23 days
Weighted average remaining contractual term, exercisable (in years)	4 years 6 months 28 days
Options, outstanding, intrinsic value | $	$ 38.2
Options, vested and expected to vest, intrinsic value | $	33.3
Options, vested and expected to vest, exercisable | $	$ 12.1