STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Net change in cash flow hedges:
Natural gas, tax expense (benefit)	$ 2,584	$ (3,191)
Interest rate, tax expense	80	105
Other postretirement benefits liability adjustment, tax (benefit) expense	$ 510	$ 193