Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Other Commitments [Line Items]
Land agreements, rent expense	$ 4	$ 4	$ 12	$ 13
Cash paid for contingent consideration			21	$ 0
Turbine Manufacturer
Other Commitments [Line Items]
Loss contingency accrual	2		2
Power Sale Agreements
Other Commitments [Line Items]
Letters of credit issued	153		153
Project Finance Agreements
Other Commitments [Line Items]
Letters of credit issued	161		161
Broadview Project
Other Commitments [Line Items]
Contingent liabilities	50		50
Broadview Project | Continued Operation Of Broadview
Other Commitments [Line Items]
Contingent liability	7		7
Cash paid for contingent consideration			1
Broadview Project | Continued Operation Of Grady
Other Commitments [Line Items]
Contingent liability	29		29
Panhandle 2
Other Commitments [Line Items]
Loss contingency accrual	$ 4		$ 4
Panhandle 2 | Maximum
Other Commitments [Line Items]
Loss contingency accrual					$ 5
Interest in Western Interconnect by Third Party
Other Commitments [Line Items]
Ownership percentage	1.00%		1.00%
Collectibility of Receivables | Pacific Gas & Electric
Other Commitments [Line Items]
Pre-petition invoice amount			$ 2
Measurement Input, Discount Rate | Broadview Project
Other Commitments [Line Items]
Contingent consideration, measurement input	0.05		0.05