Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions	Jun. 30, 2017 USD ($) $ / MWh	Dec. 31, 2016 USD ($) $ / MWh
Assets
Derivative assets	$ 870	$ 1,248
Liabilities
Derivative liabilities	1,004	1,376
Fair value, measurements, recurring
Liabilities
Derivative liabilities	1,004	1,376
Fair value, measurements, recurring | Level 3
Liabilities
Derivative liabilities	121	158
Commodity contracts
Assets
Derivative assets	831	1,199
Liabilities
Derivative liabilities	909	1,288
Commodity contracts | Fair value, measurements, recurring
Assets
Derivative assets	831	1,199
Liabilities
Derivative liabilities	909	1,288
Commodity contracts | Fair value, measurements, recurring | Level 3
Assets
Derivative assets	110	90
Liabilities
Derivative liabilities	121	158
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts
Assets
Derivative assets	57	39
Liabilities
Derivative liabilities	$ 78	$ 108
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	10	11
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	115	104
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	30	31
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs
Assets
Derivative assets	$ 53	$ 51
Liabilities
Derivative liabilities	$ 43	$ 50
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | Low
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	(29)	(22)
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | High
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	27	17
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | Weighted Average
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	0	0