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, 2016	Mar. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 5,060,000	$ 15,884,000
Purchases	1,843,000	4,928,000
Settlements	(13,219,000)	(8,379,000)
Gains (losses) recognized as regulatory assets and liabilities	8,046,000	(5,976,000)
Balance at end of period	1,730,000	6,457,000
Transfers into Level 3	0	0
Transfers out of Level 3	$ 0	$ 0