Fair Value Measurements - Net Change in Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category (Details) - 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]
Unrealized gains or losses included in earnings in the Level 3 fair value category	$ 0	$ 0	$ 0	$ 0
Commodity
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	0	0	0	0
Total realized and unrealized gains (losses), included in regulatory assets/ liabilities	1,600,000	0	1,600,000	0
Ending balance	$ 1,600,000	$ 0	$ 1,600,000	$ 0