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
	Mar. 31, 2015	Mar. 31, 2014
Commodity Contract
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 15,884,000	$ 9,933,000
Purchases	4,928,000	1,056,000
Settlements	(8,379,000)	(1,101,000)
Losses recognized as regulatory assets and liabilities	(5,976,000)	(4,097,000)
Balance at end of period	6,457,000	5,791,000
Transfers into Level 3	0	0
Transfers out of Level 3	$ 0	$ 0