Fair Value of Financial Assets and Liabilities 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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 35,389,000	$ 28,665,000	$ 12,723,000	$ 1,955,000
Purchases	3,169,000	43,000	22,517,000	39,376,000
Settlements	(10,068,000)	(9,939,000)	(35,305,000)	(40,437,000)
Net gains recognized as regulatory assets and liabilities	(3,213,000)	1,669,000	25,342,000	19,544,000
Balance at end of period	25,277,000	20,438,000	25,277,000	20,438,000
Transfers into Level 3	0	0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0