Consolidated Balance Sheets Parenthetical - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 5,989	$ 5,025
Common stock stated value per share (USD per share)	$ 20	$ 20
Authorized	50,000,000	50,000,000
Issued to voting trustees	15,912,467	14,606,830
Issued to shareholders	3,496,114	2,850,551
In treasury, at cost	(24,243)	(18,854)
Outstanding Common Stock	19,384,338	17,438,527
Common Stock, Shares Subscribed	923,156	896,456
Subscripton Receivable	(923,156)	(896,456)