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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 24,517,000	$ 46,826,000	$ 18,028,000	$ 56,155,000
Purchases		274,000	486,000	33,296,000	63,724,000
Settlements		(33,982,000)	(20,216,000)	(60,707,000)	(57,462,000)
(Losses) gains recognized in earnings	[1]	9,000	121,000	(33,000)	1,401,000
Gains (losses) recognized as regulatory assets and liabilities		33,777,000	3,966,000	34,011,000	(32,635,000)
Balance at end of period		24,595,000	31,183,000	24,595,000	31,183,000
Transfers into Level 3		0	0	0	0
Transfers out of Level 3		$ 0	$ 0	$ 0	$ 0

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