Shareholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 13, 2018
Class of Stock [Line Items]
Treasury Shares		20,733,010	19,303,578
Additional authorized shares purchased		7,162,279
Stock repurchased during period	$ 306
Repurchased shares, amount		$ (62,550)	$ (34,525)
Preferred stock, authorized (in shares)		1,000,000
Preferred stock, issued (in shares)		0
Treasury Stock
Class of Stock [Line Items]
Repurchased shares, amount		$ (62,550)	(34,525)
Additional Paid-in Capital
Class of Stock [Line Items]
Repurchased shares, amount		$ 0	$ 0
Nonvoting Common Stock
Class of Stock [Line Items]
Common stock repurchased (in shares)		1,961,442	1,372,700	0
Subsequent Event
Class of Stock [Line Items]
Authorized stock repurchase program, amount					$ 500,000