Stock-Based Compensation - Nonvested Performance-based Award Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Non-Employee Directors | Deferred Compensation, Share-based Payments
Non-vested PSU Awards [Roll Forward]
Granted at target level (in shares)	10,302
Non-vested PSU Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Granted at target level (in dollars per share)	$ 194.29
Incentive Stock Awards
Non-vested PSU Awards [Roll Forward]
Granted at target level (in shares)	350	1,300	1,500
Forfeited (in shares)	0	0	(200)
Non-vested PSU Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Granted at target level (in dollars per share)	$ 156.49	$ 106.14	$ 93.00
Stock-settled PSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares to be issued upon conversion (in shares)	92,792
Average remaining term of shares issued at conversion	1 year
Non-vested PSU Awards [Roll Forward]
Outstanding, beginning (in shares)	264,622	296,037	341,944
Granted at target level (in shares)	53,659	84,309	102,307
Adjustments above/(below) target (in shares)	(14,004)	(50,556)	(2,284)
Vested and converted (in shares)	(70,074)	(48,964)	(121,984)
Forfeited (in shares)	(11,404)	(16,204)	(23,946)
Outstanding, ending (in shares)	222,799	264,622	296,037
Non-vested PSU Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, beginning (in dollars per share)	$ 92.80	$ 76.84	$ 64.38
Granted at target level (in dollars per share)	177.31	103.40	90.45
Adjustments above/(below) target (in dollars per share)	77.02	83.89	33.86
Vested and converted (in dollars per share)	173.22	102.51	93.00
Forfeited (in dollars per share)	104.43	69.09	68.65
Outstanding, ending (in dollars per share)	$ 116.37	$ 92.80	$ 76.84
Cash-Settled PSU Awards
Non-vested PSU Awards [Roll Forward]
Outstanding, beginning (in shares)	1,981	1,592	1,972
Granted at target level (in shares)	732	806	560
Adjustments above/(below) target (in shares)	(99)	(206)	(30)
Vested and converted (in shares)	(502)	(211)	(910)
Forfeited (in shares)	0	0	0
Outstanding, ending (in shares)	2,112	1,981	1,592
Non-vested PSU Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, beginning (in dollars per share)	$ 93.28	$ 79.48	$ 92.25
Granted at target level (in dollars per share)	190.71	102.51	89.64
Adjustments above/(below) target (in dollars per share)	74.43	56.95	41.53
Vested and converted (in dollars per share)	173.22	102.51	93.00
Forfeited (in dollars per share)	0	0	0
Outstanding, ending (in dollars per share)	$ 130.13	$ 93.28	$ 79.48
Restricted Stock
Non-vested PSU Awards [Roll Forward]
Granted at target level (in shares)	8,721	13,308	15,942
Non-vested PSU Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Granted at target level (in dollars per share)	$ 200.35	$ 116.39	$ 96.77
Minimum | Stock-settled PSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	0.00%
Maximum | Stock-settled PSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	200.00%