Condensed Consolidated Balance Sheets Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 6,118	$ 5,989
Common, stated value per share	$ 20	$ 20
Authorized	50,000,000	50,000,000
Issued to voting trustees	16,402,110	15,912,467
Issued to shareholders	3,608,067	3,496,114
In treasury, at cost	(481,363)	(24,243)
Outstanding common stock	19,528,814	19,384,338