Fair Value Measurements - Additional Information (Details) - Clean Energy Businesses - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Non-utility revenue
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 commodity derivative assets and liabilities	$ 0	$ 0	$ 0	$ 0
Change in fair value relating to Level 3 commodity derivative assets and liabilities	0	0	0	0
Purchased power expense
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 commodity derivative assets and liabilities	4	(5)	3	(6)
Change in fair value relating to Level 3 commodity derivative assets and liabilities	$ 4	$ (4)	$ 2	$ (2)