Share-based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	1.97%	1.60%	1.92%
Expected life of option (in years)	3 years 7 months 6 days	3 years 7 months 6 days	4 years 1 month 6 days
Risk-free interest rate (as a percent)	1.43%	1.29%	0.96%
Expected stock price volatility (as a percent)	32.34%	33.97%	36.12%
Options Outstanding
Balance at beginning of year (in shares)	1,462,726	1,765,034	1,201,000
Granted (in shares)	389,836	102,050	828,100
Exercised (in shares)	(217,438)	(348,156)	(223,531)
Lapsed or cancelled (in shares)	(40,117)	(56,202)	(40,535)
Balance at end of period (in shares)	1,595,007	1,462,726	1,765,034
Options exercisable at end of period (in shares)	589,055	428,951	461,458
Weighted average fair value of options granted during the period	$ 9,522,613	$ 2,841,678	$ 18,041,576
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 87.98	$ 83.24	$ 75.05
Granted (in dollars per share)	112.54	119.12	87.23
Exercised (in dollars per share)	74.69	72.76	53.93
Lapsed or cancelled (in dollars per share)	98.61	90.03	83.94
Balance at end of period (in dollars per share)	95.52	87.98	83.24
Options exercisable at end of period (in dollars per share)	$ 89.85	$ 90.32	$ 89.38
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 16,061,201	$ 1,141,675	$ 17,386,949
Summary of restricted stock
Balance at beginning of year (in shares)	3,000,979	2,994,197	2,804,901
Granted (in shares)	143,053	9,550	192,563
Cancelled (in shares)	(6,151)	(2,768)	(3,267)
Balance at end of period (in shares)	3,137,881	3,000,979	2,994,197
Vested during the period (in shares)	87,081	75,043	21,074
Compensation expense recorded	$ 7,540,747	$ 9,658,019	$ 6,713,155