Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 21, 2017	May 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Mar. 07, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Development expenses					$ 7	$ (3)
Tsugaru Holdings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additional earn-out payment possible								$ 118
Contingent consideration								106
Tsugaru Holdings | Term Conversion Of Tsugaru Construction Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase in additional earn-out payment			$ 10		10			10
Decrease in additional earn-out payment								10
Contingent consideration			1		1			$ 103
Development expenses			9
Broadview Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	$ 21
Development expenses			4
Transmission wheeling revenue, period	25 years
Energy derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset, change in unrealized gain (loss)			0	$ 0	(7)	(15)
Pattern Energy Group LP | Broadview Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration paid		$ 25
Recurring Measurement Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration			122		122		$ 130
Recurring Measurement Basis | Contingent Consideration | Other Income (Expense), Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability, change in unrealized gain (loss)			$ (2)	$ (2)	$ (6)	$ (7)