Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 56,155,000	$ 41,660,000	$ 16,649,000
Purchases		63,712,000	135,008,000	61,474,000
Settlements		(69,754,000)	(145,974,000)	(45,199,000)
Gains recognized in earnings	[1]	1,533,000	10,692,000	3,947,000
(Losses) gains recognized as regulatory assets and liabilities		(33,618,000)	15,862,000	4,789,000
Balance at end of period		18,028,000	56,155,000	41,660,000
Transfers into Level 3		0	0	0
Transfers out of Level 3		$ 0	$ (1,093,000)	$ 0

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