Capital Stock - Captial Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Equity [Abstract]
Percent of entity owned by current and retired employees			100.00%
Percent of entity held in voting trust	81.00%		81.00%
Common, stated value (in dollars per share)	$ 20		$ 20		$ 20
Dividends, Common Stock, Cash	$ 5,318	$ 4,994	$ 10,624	$ 9,977
Preferred Stock, Shares Authorized	10,000,000		10,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Preferred Stock, Shares Outstanding	0		0		0