Statements of Consolidated Comprehensive (Loss) Income (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Natural gas, tax benefit	$ (9,894)	$ (26,093)	$ (28,934)	$ (81,279)
Interest rate, tax expense	26	25	78	75
Pension and other post-retirement benefits liability adjustment, tax expense	$ 52	$ 127	$ 6,287	$ 382