Shareholders' Equity (Narrative) (Details) - USD ($)			12 Months Ended		17 Months Ended
	Oct. 30, 2017	Jul. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, repurchased cost			$ 80,400,000	$ 43,800,000
Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, expiration date		Jul. 31, 2018
Stock repurchase program, additional authorized amount	$ 250,000,000
Shares repurchase program, extended expiration date	Oct. 31, 2020
Shares repurchase program, shares repurchased			1,841,690	1,111,525	2,950,000
Shares repurchase program, repurchased cost			$ 80,400,000	$ 43,800,000	$ 124,200,000
Shares repurchase program, average price per share			$ 43.58	$ 39.45	$ 42.03
Maximum [Member] | Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, authorized amount		$ 150,000,000