Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
		Feb. 28, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Performance Shares [Member]
Stock-based Compensation [Line Items]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition			1 year 7 months 6 days
Number of Shares [Rollforward]
As of January 1			99,585	129,693	127,898
Granted	[1]		25,763	60,747	66,557
Awarded			(25,304)	(75,943)	(58,293)
Forfeited			(1,135)	(14,912)	(6,469)
As of December 31			85,284	99,585	129,693
Performance Period (Years)			3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 72.78	$ 66.12	$ 58.23
Granted	[1]		83.17	63.89	76.42
Awarded			62.03	53.44	59.82
Forfeited			79.81	77.14	72.99
As of December 31			$ 80.73	$ 72.78	$ 66.12
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Unearned Grant Award			13,625	0	0
Unearned Grant Award			$ 62.03	$ 0	$ 0
Restricted Stock Units [Member]
Stock-based Compensation [Line Items]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition			1 year 8 months 12 days
Number of Shares [Rollforward]
As of January 1			39,943	49,771	55,248
Granted	[2]		15,169	13,927	16,573
Awarded			(17,193)	(21,110)	(18,881)
Forfeited			(437)	(2,645)	(3,169)
As of December 31			37,482	39,943	49,771
Performance Period (Years)			3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 69.30	$ 60.74	$ 56.18
Granted	[2]		83.48	74.93	71.11
Awarded			63.41	52.44	55.78
Forfeited			77.52	72.43	64.92
As of December 31			$ 77.64	$ 69.30	$ 60.74
Subsequent Event [Member] | Performance Shares [Member]
Number of Shares [Rollforward]
Granted		30,000
Awarded		0
Performance Period (Years)		3 years
Granted, Grant Date Fair Value		$ 2.2
Subsequent Event [Member] | Restricted Stock Units [Member]
Number of Shares [Rollforward]
Granted		14,000
Awarded		(12,000)
Granted, Grant Date Fair Value		$ 0.9
Awarded, Grant Date Fair Value		$ 0.9

[1] Shares granted include accrued dividends.

[2] Shares granted include accrued dividends.