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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 1,730,000	$ 6,457,000	$ 5,060,000	$ 15,884,000
Purchases	1,956,000	17,284,000	3,800,000	22,213,000
Settlements	(7,130,000)	(10,022,000)	(14,122,000)	(18,400,000)
Gains (losses) recognized as regulatory assets and liabilities	4,514,000	(2,256,000)	6,332,000	(8,234,000)
Balance at end of period	1,070,000	11,463,000	1,070,000	11,463,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0