Share-based Compensation - Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)			2.85%
Expected life of option (in years)			3 years 6 months
Risk-free interest rate (as a percent)			2.48%
Expected stock price volatility (as a percent)			22.00%
Options Outstanding
Balance at beginning of year (in shares)	1,007,665	1,104,780	1,504,809
Granted (in shares)	0	0	5,830
Exercised (in shares)	0	0	(307,334)
Lapsed or cancelled (in shares)	(222,670)	(97,115)	(98,525)
Balance at end of period (in shares)	784,995	1,007,665	1,104,780
Options exercisable at end of period (in shares)	784,022	888,988	760,834
Weighted average fair value of options granted during the period	$ 0	$ 0	$ 84,068
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 105.35	$ 106.56	$ 104.44
Granted (in dollars per share)	0	0	100.77
Exercised (in dollars per share)	0	0	92.85
Lapsed or cancelled (in dollars per share)	114.97	119.19	116.52
Balance at end of period (in dollars per share)	102.62	105.35	106.56
Options exercisable at end of period (in dollars per share)	$ 102.62	$ 104.66	$ 104.24
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 734,315	$ 11,131,181	$ 13,440,503
Summary of restricted stock
Balance at beginning of year (in shares)	3,465,347	3,354,142	3,204,703
Granted (in shares)	8,959	122,768	158,281
Cancelled (in shares)	(34,632)	(11,563)	(8,842)
Balance at end of period (in shares)	3,439,674	3,465,347	3,354,142
Vested during the period (in shares)	128,891	110,048	89,502
Compensation expense recorded	$ 10,895,459	$ 12,892,249	$ 12,757,704