Capital Stock - Captial Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Equity [Abstract]
Percent of entity owned by current and retired employees	100.00%
NumberofShareholdersInVotingTrust	4,906
CommonStockSharesVotingTrustees	14,179,701
Percent of entity held in voting trust	80.00%
Common, stated value (in dollars per share)	$ 20		$ 20
Dividends, Common Stock, Cash	$ 5,306	$ 4,983
Preferred Stock, Shares Authorized	10,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Shares Outstanding	0		0