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, 2017 USD ($) MWh MMBTU mw_per_day T shares bbl	Dec. 31, 2016 USD ($) MWh MMBTU mw_per_day T shares bbl
Short | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass	2
Short | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure | MMBTU		53
Short | Capacity
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, flow rate | mw_per_day	1	1
Long | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass		0
Long | Coal
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass	22	35
Long | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure | MMBTU	45
Long | Oil
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, volume | bbl	0	1
Long | Power
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure | MWh	24	7
Long | Interest
Volumetric Underlying Derivative Transactions
Derivative, notional amount (in usd) | $	$ 3,701	$ 3,429
Long | Equity
Volumetric Underlying Derivative Transactions
Derivative, non-monetary notional amount (in shares) | shares	1	1