Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 12,970,000	$ 40,271,000
Purchases		0	864,000
Settlements		(5,038,000)	(11,552,000)
(Losses) gains recognized in earnings	[1]	(24,000)	60,000
(Losses) recognized as regulatory assets and liabilities		(2,784,000)	(18,671,000)
Balance at end of period		5,124,000	10,972,000
Transfers into Level 3		0	0
Transfers out of Level 3		$ 0	$ 0

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