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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 1,070,000	$ 11,463,000	$ 5,060,000	$ 15,884,000
Purchases	274,000	408,000	5,426,000	22,621,000
Settlements	(7,822,000)	(7,409,000)	(22,438,000)	(25,810,000)
Gains (losses) recognized as regulatory assets and liabilities	6,614,000	2,898,000	12,088,000	(5,335,000)
Balance at end of period	136,000	7,360,000	136,000	7,360,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0