Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions	Sep. 30, 2019 USD ($) $ / MWh	Dec. 31, 2018 USD ($) $ / MWh
Liabilities
Derivative liabilities	$ 1,032	$ 977
Level 3
Liabilities
Derivative liabilities	110	89
Commodity contracts
Assets
Derivative assets	1,093	1,042
Liabilities
Derivative liabilities	1,032	977
Commodity contracts | Level 3
Assets
Derivative assets	123	109
Liabilities
Derivative liabilities	110	89
Commodity contracts | Level 3 | Power Contracts
Assets
Derivative assets	89	89
Liabilities
Derivative liabilities	$ 98	$ 75
Commodity contracts | Level 3 | Power Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	6	1
Commodity contracts | Level 3 | Power Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	217	214
Commodity contracts | Level 3 | Power Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	22	31
Commodity contracts | Level 3 | FTRs
Assets
Derivative assets	$ 34	$ 20
Liabilities
Derivative liabilities	$ 12	$ 14
Commodity contracts | Level 3 | FTRs | Low
Fair Value Inputs / Range
Derivative, Auction Price | $ / MWh	(134)	(90)
Commodity contracts | Level 3 | FTRs | High
Fair Value Inputs / Range
Derivative, Auction Price | $ / MWh	60	34
Commodity contracts | Level 3 | FTRs | Weighted Average
Fair Value Inputs / Range
Derivative, Auction Price | $ / MWh	0	0