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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 28,665,000	$ 1,070,000	$ 1,955,000	$ 5,060,000
Purchases	43,000	274,000	39,376,000	5,426,000
Settlements	(9,939,000)	(7,822,000)	(40,437,000)	(22,438,000)
Net gains recognized as regulatory assets and liabilities	1,669,000	6,614,000	19,544,000	12,088,000
Balance at end of period	20,438,000	136,000	20,438,000	136,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0