Statements of Consolidated Comprehensive Income (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statements of Consolidated Comprehensive Income
Natural gas, tax expense (benefit)	$ 41,436	$ (41,598)	$ (10,147)	$ (3,105)
Interest rate, tax expense (benefit)	25	(4,966)	50	(3,165)
Pension and other post-retirement benefits liability adjustment, tax expense	$ 306	$ 92	$ 613	$ 214