Share-based Compensation - Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	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)	0 days	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 year (in shares)	1,137,017	1,548,719	1,737,213
Granted (in shares)	0	6,000	174,000
Exercised (in shares)	0	(316,302)	(292,193)
Lapsed or cancelled (in shares)	(99,949)	(101,400)	(70,301)
Balance at end of period (in shares)	1,037,068	1,137,017	1,548,719
Options exercisable at end of period (in shares)	914,929	783,035	800,902
Weighted average fair value of options granted during the period	$ 0	$ 84,068	$ 3,816,652
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 103.54	$ 101.48	$ 98.44
Granted (in dollars per share)	0	97.91	105.66
Exercised (in dollars per share)	0	90.22	81.07
Lapsed or cancelled (in dollars per share)	115.81	113.22	121.68
Balance at end of period (in dollars per share)	102.36	103.54	101.48
Options exercisable at end of period (in dollars per share)	$ 101.69	$ 101.28	$ 94.33
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 11,131,181	$ 13,440,503	$ 9,905,986
Summary of restricted stock
Balance at beginning of year (in shares)	3,452,016	3,298,216	3,202,031
Granted (in shares)	126,350	162,900	96,185
Cancelled (in shares)	(11,900)	(9,100)	0
Balance at end of period (in shares)	3,566,466	3,452,016	3,298,216
Vested during the period (in shares)	113,259	92,114	95,736
Compensation expense recorded	$ 12,892,249	$ 12,757,704	$ 9,809,749