Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 11,463,000	$ 33,942,000	$ 15,884,000	$ 9,933,000
Purchases	408,000	4,429,000	22,621,000	43,904,000
Settlements	(7,409,000)	(8,346,000)	(25,810,000)	(23,001,000)
(Losses) gains recognized as regulatory assets and liabilities	2,898,000	(7,785,000)	(5,335,000)	(8,596,000)
Balance at end of period	7,360,000	22,240,000	7,360,000	22,240,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0