Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Non-utility revenue
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in fair value relating to Level 3 commodity derivative assets and liabilities	$ (1)		$ (5)
Purchased power cost
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in fair value relating to Level 3 commodity derivative assets and liabilities		$ 4		$ 2
Clean Energy Businesses | 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	(1)	0	(4)	0
Change in fair value relating to Level 3 commodity derivative assets and liabilities		0		0
Clean Energy Businesses | Purchased power cost
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 commodity derivative assets and liabilities	0	$ 4	0	$ 3
Change in fair value relating to Level 3 commodity derivative assets and liabilities	$ 0		$ 0