Share-Based Compensation - Stock Options - (Details) - USD ($)	12 Months Ended			59 Months Ended	156 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2016
Number of shares
Exercisable at the end of the period (in shares)		227,130
Stock Options
Number of shares
Outstanding at the beginning of the period (in shares)	285,405	294,342	295,916
Granted (in shares)	25,682	26,430	22,497
Exercised (in shares)	(63,527)	(35,360)	(22,878)
Forfeited (in shares)	(1,025)		(1,180)
Expired (in shares)		(7)	(13)
Outstanding at the end of the period (in shares)	246,535	285,405	294,342	246,535	246,535
Exercisable at the end of the period (in shares)	185,758		239,417	185,758	185,758
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 38.85	$ 35.91	$ 33.26
Granted (in dollars per share)	63.79	61.74	65.59
Exercised (in dollars per share)	34.70	31.51	31.06
Forfeited (in dollars per share)	35.20		31.97
Expired (in dollars per share)		31.72	27.22
Outstanding at the end of the period (in dollars per share)	42.53	38.85	35.91	$ 42.53	$ 42.53
Exercisable at the end of the period (in dollars per share)	36.33	34.48	33.17	$ 36.33	$ 36.33
Weighted-average fair value of options granted during the year (in dollars per share)	$ 25.19	$ 25.08	$ 26.44
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 11,100,000	$ 9,400,000	$ 9,200,000	$ 11,100,000	$ 11,100,000
Exercisable at the end of the period	9,500,000	8,500,000	8,100,000	$ 9,500,000	$ 9,500,000
Exercised at the end of the period	$ 2,500,000	$ 1,300,000	$ 724,000
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	1,684,000
2012 plan | Restricted Stock. | Maximum
Share-Based Compensation
Number of shares registered under the 2012 plan	817,476			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