Share-Based Compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended			59 Months Ended	168 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2017
Number of shares
Exercisable at the end of the period (in shares)		185,758		185,758
Stock Options
Number of shares
Outstanding at the beginning of the period (in shares)	246,535	285,405	294,342
Granted (in shares)	33,634	25,682	26,430
Exercised (in shares)	(59,480)	(63,527)	(35,360)
Forfeited (in shares)	(2,000)	(1,025)
Expired (in shares)			(7)
Outstanding at the end of the period (in shares)	218,689	246,535	285,405	246,535	218,689
Exercisable at the end of the period (in shares)	148,702		227,130		148,702
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 42.53	$ 38.85	$ 35.91
Granted (in dollars per share)	91.23	63.79	61.74
Exercised (in dollars per share)	33.03	34.70	31.51
Forfeited (in dollars per share)	26.01	35.20
Expired (in dollars per share)			31.72
Outstanding at the end of the period (in dollars per share)	52.75	42.53	38.85	$ 42.53	$ 52.75
Exercisable at the end of the period (in dollars per share)	41.62	36.33	34.48	$ 36.33	$ 41.62
Weighted-average fair value of options granted during the year (in dollars per share)	$ 35.42	$ 25.19	$ 25.08
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 7.7	$ 11.1	$ 9.4	$ 11.1	$ 7.7
Exercisable at the end of the period	6.8	9.5	8.5	$ 9.5	$ 6.8
Exercised at the end of the period	$ 3.3	$ 2.5	$ 1.3
Incentive stock options
Share-Based Compensation
Expiration period	10 years
Incentive stock options | Maximum
Share-Based Compensation
Vesting period	4 years
2012 plan
Share-Based Compensation
Number of shares registered under the 2012 plan	1,684,000				1,684,000
2012 plan | Restricted Stock. | Maximum
Share-Based Compensation
Number of shares registered under the 2012 plan	817,476				817,476
1999, 2004 and 2012 Plans | Incentive stock options
Share-Based Compensation
Vesting percentage	25.00%
1999 plan
Share-Based Compensation
Options granted (in shares)					0
2004 plan
Share-Based Compensation
Options granted (in shares)				0