STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net change in cash flow hedges:
Natural gas, tax expense (benefit)	$ (102,271)	$ 102,850	$ (50,200)
Interest rate, tax expense	100	104	63
Pension and other post-retirement benefits liability adjustment, tax (benefit) expense	$ (564)	$ (515)	$ 16,115