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		$ 18,028,000	$ 56,155,000
Purchases		1,843,000	5,792,000
Settlements		(18,256,000)	(19,931,000)
Losses (gains) recognized in earnings	[1]	(24,000)	60,000
Gains (losses) recognized as regulatory assets and liabilities		5,263,000	(24,647,000)
Balance at end of period		6,854,000	17,429,000
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.