Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Treasury Shares	23,567,979	20,733,010
Authorized stock repurchase program, amount	$ 331,300
Stock repurchased during period	387
Repurchased shares, amount	$ (168,735)	$ (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	$ (168,735)	$ (62,550)	$ (34,525)
Nonvoting Common Stock
Class of Stock [Line Items]
Common stock repurchased (in shares)	3,749,493	1,961,442	1,372,700