Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) (Details) - Commodity Contract - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 1,955,000	$ 5,060,000	$ 15,884,000
Purchases	41,176,000	7,616,000	23,425,000
Settlements	(55,758,000)	(41,923,000)	(31,703,000)
Gains (losses) recognized as regulatory assets	25,350,000	31,202,000	(2,546,000)
Balance at end of period	12,723,000	1,955,000	5,060,000
Transfers into Level 3	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0