STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net change in cash flow hedges:
Natural gas, tax expense (benefit)	$ (3,191)	$ (36,296)	$ (102,271)
Interest rate, tax expense	105	104	100
Pension and other post-retirement benefits liability adjustment, tax (benefit) expense	$ 193	$ 6,778	$ (564)