Stock-Based Compensation Plans (Summary Of Restricted Stock Units Activity) (Details) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, as of December 31		455,823
Granted		108,100
Converted		(34,836)
Forfeited		6,260
Outstanding, as of December 31		522,827	[1]	455,823
Vested		258,195
Weighted Average Grant-Date Fair Value Per Share, Outstanding as of December 31		$ 101.75
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Granted		206.45		$ 181.04	$ 139.00
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Converted		40.12
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Forfeited		178.07
Weighted Average Grant-Date Fair Value Per Share, Outstanding as of December 31		126.59		$ 101.75
Weighted Average Grant-Date Fair Value Per Share, Vested as of December 31		$ 106.67
Aggregate Intrinsic Value, Outstanding	[2]	$ 169.1
Aggregate Intrinsic Value, Vested	[2]	$ 83.5
Weighted Average Remaining Contractual Term, Outstanding		5 years 2 months
Weighted Average Remaining Contractual Term, Vested		4 years 2 months

[1] No RSUs outstanding at December 31, 2017 were convertible to shares of common stock.

[2] The intrinsic value of RSUs is measured by applying the closing stock price as of December 31, 2017 to the applicable number of units.