Statements of Consolidated Comprehensive Income (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Natural gas, net of tax benefit of $(1,101), $(10,701), $(1,685), and $(19,040)	$ (1,101)	$ (10,701)	$ (1,685)	$ (19,040)
Interest rate, net of tax expense of $27, $27, $52, and $52	27	27	52	52
Pension and other post-retirement benefits liability adjustment, net of tax expense of $49, $6,100, $98, and $6,235	$ 49	$ 6,100	$ 98	$ 6,235