Share-based Compensation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	1.60%	1.92%	2.00%
Expected life of option	3 years 7 months 6 days	4 years 1 month 6 days	3 years 8 months 12 days
Risk-free interest rate (as a percent)	1.29%	0.96%	0.46%
Expected stock price volatility (as a percent)	33.97%	36.12%	37.40%
Options Outstanding
Balance at beginning of year (in shares)	1,765,034	1,201,000	1,277,200
Granted (in shares)	102,050	828,100	361,331
Exercised (in shares)	(348,156)	(223,531)	(382,612)
Lapsed or cancelled (in shares)	(56,202)	(40,535)	(54,919)
Balance at end of period (in shares)	1,462,726	1,765,034	1,201,000
Options exercisable at end of period (in shares)	428,951	461,458	479,913
Weighted average fair value of award granted during the year	$ 2,841,678	$ 18,041,576	$ 6,602,967
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 83.24	$ 75.05	$ 63.37
Granted (in dollars per share)	$ 119.12	$ 87.23	$ 75.36
Exercised (in dollars per share)	$ 72.76	$ 53.93	$ 36.65
Lapsed or cancelled (in dollars per share)	$ 90.03	$ 83.94	$ 72.99
Balance at end of period (in dollars per share)	$ 87.98	$ 83.24	$ 75.05
Options exercisable at end of period (in dollars per share)	$ 90.32	$ 89.38	$ 86.85
Restricted Stock Awards
Options Outstanding
Weighted average fair value of award granted during the year	1,141,675	17,386,949	7,023,942
Summary of restricted stock
Balance at beginning of year (in shares)	2,994,197	2,804,901	2,912,456
Granted (in shares)	9,550	192,563	92,729
Cancelled (in shares)	(2,768)	(3,267)	(200,284)
Balance at end of period (in shares)	3,000,979	2,994,197	2,804,901
Vested during the period (in shares)	75,043	21,074	408,800
Compensation expense recorded	$ 9,658,019	$ 6,713,155	$ 6,930,381