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, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Commodity Contract
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 24,217,000		$ 7,642,000		$ 41,660,000		$ 16,649,000
Purchases	120,107,000		51,386,000		121,164,000		51,386,000
Settlements	(33,610,000)		(8,503,000)		(87,419,000)		(20,952,000)
Gains (losses) recognized in earnings	6,438,000	[1]	(217,000)	[1]	7,437,000	[1]	(279,000)	[1]
Gains (losses) recognized as regulatory assets and liabilities	(11,758,000)		(3,090,000)		22,552,000		414,000
Balance at end of period	105,394,000		47,218,000		105,394,000		47,218,000
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	$ 0		$ 0		$ 0		$ 0

[1] These amounts relate to commodity derivatives held at the end of the period.