Common Stock Repurchase Plan (Narrative) (Details) - USD ($)			3 Months Ended		20 Months Ended
	Oct. 30, 2017	Jul. 29, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, repurchased cost			$ 70,000,000	$ 50,000,000
Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, expiration date		Jul. 31, 2018
Shares repurchase program, shares repurchased			1,165,247	1,190,021	4,100,000
Shares repurchase program, repurchased cost			$ 70,000,000		$ 194,200,000
Shares repurchase program, average price per share			$ 60.10		$ 47.15
Stock repurchase program, additional authorized amount	$ 250,000,000
Shares repurchase program, extended expiration date	Oct. 31, 2020
Common Stock [Member] | Maximum [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, authorized amount		$ 150,000,000