Stock-based Compensation Plans (Summary of Restricted Stock Activity) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Time-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		335,166	332,481	298,528
Granted, Shares		143,128	149,698	152,529
Vested, Shares		(94,993)	(109,493)	(72,710)
Forfeited, Shares	[1]	(32,821)	(37,520)	(45,866)
Outstanding, end of period, Shares		350,480	335,166	332,481
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 109.55	$ 97.28	$ 83.45
Granted, Weighted Average Grant Date Fair Value		117.72	117.60	111.66
Vested, Weighted Average Grant Date Fair Value		94.54	84.57	72.69
Forfeited, Weighted Average Grant Date Fair Value		115.74	106.03	94.04
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 116.37	$ 109.55	$ 97.28
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		86,252	69,838	102,586
Granted, Shares		54,415	42,605	35,063
Vested, Shares		(40,601)	(23,521)	(14,032)
Forfeited, Shares	[1]	(3,023)	(2,670)	(53,779)
Outstanding, end of period, Shares		97,043	86,252	69,838
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 110.70	$ 95.58	$ 81.21
Granted, Weighted Average Grant Date Fair Value		105.10	116.67	118.60
Vested, Weighted Average Grant Date Fair Value		80.92	79.57	70.24
Forfeited, Weighted Average Grant Date Fair Value		124.78	135.92	89.79
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 119.59	$ 110.70	$ 95.58

[1] As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees.