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, 2017	Mar. 31, 2016
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
Purchases	3,511,000	1,843,000
Settlements	(16,400,000)	(13,219,000)
Gains (losses) recognized as regulatory assets and liabilities	12,126,000	8,046,000
Balance at end of period	1,192,000	1,730,000
Transfers into Level 3	0	0
Transfers out of Level 3	$ 0	$ 0