Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) shares in Millions, mw_per_day in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) MWh MMBTU mw_per_day T shares bbl	Dec. 31, 2018 USD ($) MWh MMBTU mw_per_day T shares bbl
Short | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton)	1	2
Short | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MMBTU	238	330
Short | Capacity
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, flow rate (in MW/Day) | mw_per_day	(1)	(1)
Long | Renewable Energy Certificates
Volumetric Underlying Derivative Transactions
Derivative, non-monetary notional amount (in shares) | shares	1	1
Long | Coal
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton)	7	13
Long | Oil
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount (in barrels)) | bbl	0	1
Long | Power
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MWh	21	1
Long | Interest
Volumetric Underlying Derivative Transactions
Derivative, notional amount (in usd) | $	$ 0	$ 1,000