Share-Based Compensation - Stock Options - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of shares
Exercisable at the end of the period (in shares)		239,417
Stock Options
Number of shares
Outstanding at the beginning of the period (in shares)	294,342	295,916	340,140
Granted (in shares)	26,430	22,497	23,007
Exercised (in shares)	(35,360)	(22,878)	(65,555)
Forfeited (in shares)		(1,180)	(220)
Expired (in shares)	(7)	(13)	(1,456)
Outstanding at the end of the period (in shares)	285,405	294,342	295,916
Exercisable at the end of the period (in shares)	227,130		233,566
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 35.91	$ 33.26	$ 31.79
Granted (in dollars per share)	61.74	65.59	41.45
Exercised (in dollars per share)	31.51	31.06	28.75
Forfeited (in dollars per share)		31.97	27.22
Expired (in dollars per share)	31.72	27.22	22.17
Outstanding at the end of the period (in dollars per share)	38.85	35.91	33.26
Exercisable at the end of the period (in dollars per share)	34.48	33.17	32.66
Weighted-average fair value of options granted during the year (in dollars per share)	$ 25.08	$ 26.44	$ 15.66
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 9,400,000	$ 9,200,000	$ 9,800,000
Exercisable at the end of the period	8,500,000	8,100,000	7,900,000
Exercised at the end of the period	$ 1,300,000	$ 724,000	$ 1,800,000
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
2012 plan | Restricted Stock | Maximum
Share-Based Compensation
Number of shares registered under the 2012 plan	817,476
1999, 2004 and 2012 Plans | Incentive stock options
Share-Based Compensation
Vesting percentage	25.00%