Common Stock (Common Stock Repurchase Program) (Details) $ in Thousands	12 Months Ended	228 Months Ended
	Dec. 31, 2017 shares	Dec. 31, 2017 USD ($) shares
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	0	25,406,184	[1],[2]
Cost, including commission | $		$ 423,647	[1]
Total remaining shares available for repurchase at December 31, 2017	393,816	393,816
Treasury shares acquired outside of repurchase programs		86,735
Stock Issued During Period, Shares, Treasury Stock Reissued	256,929

[1] Represents repurchased shares and cost since inception of the stock repurchase program in 1999.

[2] Shares repurchased does not include 86,735 treasury shares related to employee compensation arrangements outside of the Company's repurchase programs. Beginning in 2015, shares of the Company's common stock issued for employee benefit and stock incentive plans have been issued from the shares repurchased and held in treasury stock. The Company awarded 256,929 shares, net of shares withheld for taxes, out of treasury stock during 2017.