Stock-based Compensation Plans (Summary of Restricted Stock Activity) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		298,528		269,063		301,897
Granted, Shares		152,529		130,075		143,107
Vested, Shares		(72,710)		(70,590)		(139,183)
Forfeited, Shares	[1]	(45,866)		(30,020)		(36,758)
Outstanding, end of period, Shares		332,481		298,528		269,063
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 83.45		$ 73.91		$ 54.54
Granted, Weighted Average Grant Date Fair Value		111.66		90.40		83.43
Vested, Weighted Average Grant Date Fair Value		72.69		59.29		42.97
Forfeited, Weighted Average Grant Date Fair Value		94.04		84.88		69.07
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 97.28		$ 83.45		$ 73.91
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		102,586	[2]	115,057	[2]	196,142
Granted, Shares		35,063		60,101		126,796
Vested, Shares		(14,032)		(17,642)		(144,141)
Forfeited, Shares	[1]	(53,779)		(54,930)		(63,740)
Outstanding, end of period, Shares	[2]	69,838		102,586		115,057
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 81.21		$ 78.82		$ 47.89
Granted, Weighted Average Grant Date Fair Value		118.60		79.48		73.42
Vested, Weighted Average Grant Date Fair Value		70.24		56.45		40.95
Forfeited, Weighted Average Grant Date Fair Value		89.79		82.27		58.54
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 95.58		$ 81.21		$ 78.82

[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.

[2] At December 31, 2018, only market-based awards were outstanding.