Share-based Compensation - Stock Options and Restricted Stock Activity (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	2.50%		2.37%
Expected life of option (in years)	4 years 4 months 24 days		3 years 8 months 15 days
Risk-free interest rate (as a percent)	1.73%		1.57%
Expected stock price volatility (as a percent)	28.21%		26.76%
Options Outstanding
Balance at beginning of period (in shares)	1,737,213	1,595,007	1,595,007
Granted (in shares)	171,000		445,100
Exercised (in shares)	(146,277)		(192,875)
Lapsed or cancelled (in shares)	(65,300)		(110,019)
Balance at end of period (in shares)	1,696,636		1,737,213
Options exercisable at end of period (in shares)	939,485		748,617
Weighted average fair value of options granted during the period	$ 3,775,639		$ 8,363,036
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 98.44	$ 95.52	$ 95.52
Granted (in dollars per share)	105.70		105.86
Exercised (in dollars per share)	84.02		76.90
Lapsed or cancelled (in dollars per share)	122.30		123.86
Balance at end of period (in dollars per share)	99.50		98.44
Options exercisable at end of period (in dollars per share)	$ 91.71		$ 87.72
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 30,813	$ 10,650,077
Summary of restricted stock
Balance at beginning of year (in shares)	3,202,031	3,137,881	3,137,881
Granted (in shares)	300	98,800
Cancelled (in shares)	0	(34,650)
Balance at end of period (in shares)	3,202,331		3,202,031
Vested during the period (in shares)	86,736	83,822