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	$ 105,394,000		$ 47,218,000		$ 41,660,000		$ 16,649,000
Purchases	5,588,000		155,000		126,752,000		51,541,000
Settlements	(20,032,000)		(9,342,000)		(107,451,000)		(30,294,000)
Transfers out of Level 3	(1,093,000)		0		(1,093,000)		0
Gains recognized in earnings	1,480,000	[1]	4,008,000	[1]	8,917,000	[1]	3,729,000	[1]
(Losses) gains recognized as regulatory assets and liabilities	(17,705,000)		(571,000)		4,847,000		(157,000)
Balance at end of period	73,632,000		41,468,000		73,632,000		41,468,000
Transfers into Level 3	$ 0		$ 0		$ 0		$ 0

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