Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 17,253,000	$ 18,028,000
Purchases		3,792,000	1,843,000
Settlements		(19,802,000)	(18,256,000)
(Losses) recognized in earnings	[1]	(794,000)	(24,000)
Net gains recognized as regulatory assets and liabilities		5,387,000	5,263,000
Balance at end of period		5,836,000	6,854,000
Transfers into Level 3		0	0
Transfers out of Level 3		$ 0	$ 0

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