Fair Value Measurements - Additional Information (Details) - Clean Energy Businesses - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	(1)	5	(1)	(2)
Change in fair value relating to Level 3 commodity derivative assets and liabilities	$ (2)	$ 7	$ (2)	$ 3