Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Feb. 15, 2016	Feb. 14, 2016	Feb. 16, 2015	Feb. 15, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 13, 2014
Common Stock, Dividends, Per Share, Declared	$ 0.19	$ 0.18	$ 0.18	$ 0.16	$ 0.76	$ 0.76	$ 0.72
Treasury stock, acquired					$ 0	$ 0	$ 303,651
Stock Repurchase Program, Remaining Authorized Repurchase Amount					$ 160,700
Share repurchase program 2015
Stock Repurchase Program, Authorized Amount								$ 500,000
Treasury stock, shares acquired					0	0
Share Repurchase Program 2014
Treasury stock, shares acquired							6,047,839
Treasury stock, acquired							$ 303,700