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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 5,343,000	$ 1,192,000	$ 12,723,000	$ 1,955,000
Purchases	18,668,000	35,822,000	19,348,000	39,333,000
Settlements	(14,798,000)	(14,098,000)	(25,237,000)	(30,498,000)
Net gains recognized as regulatory assets and liabilities	26,176,000	5,749,000	28,555,000	17,875,000
Balance at end of period	35,389,000	28,665,000	35,389,000	28,665,000
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