Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Equity [Abstract]
Percent of entity owned by current and retired employees	100.00%
Percent of entity held in voting trust	82.00%
Common, stated value (in dollars per share)	$ 20		$ 20
Dividends, Common Stock, Cash	$ 5,874	$ 5,306
Preferred Stock, Shares Authorized	10,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Shares Outstanding	0		0