Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 41,660,000
Purchases		35,593,000	63,712,000	135,008,000
Settlements		(89,085,000)	(69,754,000)	(145,974,000)
(Losses) gains recognized in earnings	[1]	(54,000)	1,533,000	10,692,000
Gains (losses) recognized as regulatory assets and liabilities		52,771,000	(33,618,000)	15,862,000
Balance at end of period		17,253,000	18,028,000	56,155,000
Transfers into Level 3		0	0	0
Transfers out of Level 3		$ 0	$ 0	$ (1,093,000)

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