Share-based Compensation - Stock Options and Restricted Stock Activity (Details) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	2.85%		2.51%
Expected life of option (in years)	3 years 6 months		4 years 4 months 24 days
Risk-free interest rate (as a percent)	2.48%		1.73%
Expected stock price volatility (as a percent)	22.00%		28.10%
Options Outstanding
Balance at beginning of period (in shares)	1,548,719	1,737,213	1,737,213
Granted (in shares)	6,000		174,000
Exercised (in shares)	(58,734)		(292,193)
Lapsed or cancelled (in shares)	(56,033)		(70,301)
Balance at end of period (in shares)	1,439,952		1,548,719
Options exercisable at end of period (in shares)	1,067,969		800,902
Total fair value of options granted during the period	$ 84,068		$ 3,816,652
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 101.48	$ 98.44	$ 98.44
Granted (in dollars per share)	97.91		105.66
Exercised (in dollars per share)	89.62		81.07
Lapsed or cancelled (in dollars per share)	114.70		121.68
Balance at end of period (in dollars per share)	101.43		101.48
Options exercisable at end of period (in dollars per share)	$ 98.93		$ 94.33
Restricted Stock Awards
Options Outstanding
Total fair value of options granted during the period	$ 163,831	$ 9,905,986
Summary of restricted stock
Balance at beginning of year (in shares)	3,298,216	3,202,031	3,202,031
Granted (in shares)	1,700	96,185
Cancelled (in shares)	(8,300)	0
Balance at end of period (in shares)	3,291,616		3,298,216
Vested during the period (in shares)	92,114	95,736