Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) (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	$ 5,060,000	$ 15,884,000	$ 9,933,000
Purchases	7,616,000	23,425,000	50,244,000
Settlements	(41,923,000)	(31,703,000)	(44,283,000)
Gains (losses) recognized as regulatory assets	31,202,000	(2,546,000)	(10,000)
Balance at end of period	1,955,000	5,060,000	15,884,000
Level 3 transfers, net	$ 0	$ 0	$ 0