Capital Stock Par Value of Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 10, 2004
Class of Stock [Line Items]
Stockholders' Equity Note, Percent of Entity Owned by Current and Retired Employees	100.00%
Common Stock, Shares, Issued, Voting Trustees	15,912,467	14,606,830
Common stock stated value per share (USD per share)	$ 20	$ 20
Common stock, subscriptions ( in shares)	923,156
Common shares subscribed (in dollars)	$ 18,463
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Shares of common stock purchased	684,347	563,590	718,269
Shares of common stock retired	678,958	610,626	666,861
Preferred stock shares authorized				10,000,000
Preferred stock par or stated value per share (USD per share)				$ 0.01
Preferred stock outstanding (in shares)	0	0
Common stock dividend declared, percent	10.00%	5.00%	2.50%
2017 Voting Trust | Common Stock
Class of Stock [Line Items]
Percentage Ownership in Voting Trust	82.00%