Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Non-utility revenue
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, assets and liabilities measured on recurring basis, change in unrealized gain (loss)	$ 2	$ 0
Purchased power expense
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, assets and liabilities measured on recurring basis, change in unrealized gain (loss)	0	(1)
Clean Energy Businesses | Non-utility revenue
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets and liabilities	2	0
Clean Energy Businesses | Purchased power expense
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets and liabilities	$ 0	$ (6)