Share-based Compensation (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	2.30%	1.97%
Expected life of option (in years)	3 years 6 months	3 years 7 months 6 days
Risk-free interest rate (as a percent)	1.27%	1.43%
Expected stock price volatility (as a percent)	31.00%	32.34%
Options Outstanding
Balance at beginning of period (in shares)	1,595,007	1,462,726
Granted (in shares)	4,500	389,836
Exercised (in shares)	(17,250)	(217,438)
Lapsed or cancelled (in shares)	(15,800)	(40,117)
Balance at end of period (in shares)	1,566,457	1,595,007
Options exercisable at end of period (in shares)	921,240	589,055
Weighted average fair value of options granted during the period	$ 84,531	$ 9,522,613
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 95.52	$ 87.98
Granted (in dollars per share)	94.75	112.54
Exercised (in dollars per share)	72.41	74.69
Lapsed or cancelled (in dollars per share)	113.59	98.61
Balance at end of period (in dollars per share)	95.59	95.52
Options exercisable at end of period (in dollars per share)	$ 88.23	$ 89.85
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 93,840	$ 16,061,201
Summary of restricted stock
Balance at beginning of year (in shares)	3,137,881	3,000,979
Granted (in shares)	1,000	143,053
Cancelled (in shares)	(4,600)	(6,151)
Balance at end of period (in shares)	3,134,281	3,137,881
Vested during the period (in shares)	81,172	87,081
Compensation expense recorded	$ 2,016,554	$ 7,540,747