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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Commodity Contract
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 33,942,000		$ 9,933,000
Purchases	4,429,000		43,904,000
Settlements	(8,346,000)		(23,001,000)
Gains (losses) recognized as regulatory assets and liabilities	(7,785,000)		(8,596,000)
Balance at end of period	22,240,000	0	22,240,000	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0