Shareholders' Equity-Common Stock Rollforward (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 01, 2016	Sep. 26, 2015	Dec. 01, 2014	May 01, 2014
Preferred Stock, Shares Authorized	2,000,000
Stock Repurchase Program, Authorized Amount				$ 400,000	$ 400,000	$ 200,000	$ 200,000
Stock Repurchase Program, Dollar Value of Shares Repurchased	$ 841,079			41,079	400,000	200,000	200,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 358,921			$ 358,921	$ 0	$ 0	$ 0
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	1,200,000
Common Stock Issued [Member]
Common stock outstanding, Beginning balance	125,424	125,424	125,424
Shares issued for stock-based compensation awards	0	0	0
Repurchases of common stock	0	0	0
Common stock outstanding, Ending balance	125,424	125,424	125,424
Treasury Stock [Member]
Common stock outstanding, Beginning balance	36,511	34,501	29,529
Shares issued for stock-based compensation awards	(1,097)	(1,372)	(1,155)
Repurchases of common stock	2,319	3,382	6,127
Common stock outstanding, Ending balance	37,733	36,511	34,501
Common Stock at Par Value [Member]
Common stock outstanding, Beginning balance	88,913	90,923	95,895
Shares issued for stock-based compensation awards	1,097	1,372	1,155
Repurchases of common stock	(2,319)	(3,382)	(6,127)
Common stock outstanding, Ending balance	87,691	88,913	90,923