CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Allowance for credit losses	$ 1,682	$ 1,691
Accumulated depreciation	$ 365,844	$ 341,119
Capital Stock, par value (in dollars per share)	$ 1	$ 1
Capital Stock, authorized shares (in shares)	50,000,000	50,000,000
Capital Stock, shares issued (in shares)	18,677,000	18,577,000
Capital Stock, shares outstanding (in shares)	18,677,000	18,577,000