Stock-Based Compensation (Stock Option Activity) (Details) - Stock Options $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Shares
Options, outstanding, beginning of period (in shares) | shares	1,272,048
Options, granted (in shares) | shares	248,899
Options, exercised (in shares) | shares	(73,741)
Options, forfeited (in shares) | shares	(6,309)
Options, outstanding, end of period (in shares) | shares	1,440,897
Options, vested and expected to vest (in shares) | shares	1,279,379
Options, exercisable (in shares) | shares	572,957
Weighted Average Exercise Price
Weighted average exercise price, beginning of period (per share) | $ / shares	$ 61.44
Weighted average exercise price, granted (per share) | $ / shares	69.12
Weighted average exercise price, exercised (per share) | $ / shares	52.72
Weighted average exercise price, forfeited (per share) | $ / shares	68.80
Weighted average exercise price, end of period (per share) | $ / shares	63.18
Weighted average exercise price, vested and expected to vest (per share) | $ / shares	64.78
Weighted average exercise price, exercisable (per share) | $ / shares	$ 77.64
Weighted average remaining contractual term (in years)	6 years 10 months 18 days
Weighted average remaining contractual term, vested and expected to vest (in years)	6 years 7 months 12 days
Weighted average remaining contractual term, exercisable (in years)	3 years 10 months
Options, outstanding, intrinsic value | $	$ 30.4
Options, vested and expected to vest, intrinsic value | $	25.5
Options, vested and expected to vest, exercisable | $	$ 6.2