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		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 6,457,000	$ 5,791,000	$ 15,884,000	$ 9,933,000
Purchases	17,284,000	38,419,000	22,213,000	39,475,000
Settlements	(10,022,000)	(13,554,000)	(18,400,000)	(14,655,000)
Losses recognized as regulatory assets and liabilities	(2,256,000)	3,286,000	(8,234,000)	(811,000)
Balance at end of period	11,463,000	33,942,000	11,463,000	33,942,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0