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
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 12,723,000	$ 1,955,000
Purchases	680,000	3,511,000
Settlements	(10,439,000)	(16,400,000)
Net gains recognized as regulatory assets and liabilities	2,379,000	12,126,000
Balance at end of period	5,343,000	1,192,000
Transfers into Level 3	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	$ 0	$ 0