Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ (5)		$ (4)
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		$ (2)		$ (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	(6)	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	$ (1)	0	$ (1)
Change in fair value relating to Level 3 commodity derivative assets and liabilities	$ 0		$ 0