Accounting for Derivative Instruments and Hedging Activities - Pre-tax effects of economic hedges not designated a cash flow hedges (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative [Line Items]
Unrealized Gain (Loss) on Cash Flow Hedging Instruments			$ 15	$ 77
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges	$ 22	$ (18)	25	(45)
Reversal of acquired loss/(gain) positions related to economic hedges	1	(2)	1	(4)
Net unrealized gains/(losses) on open positions related to economic hedges	36	(13)	15	77
Total unrealized mark-to-market gains/(losses) for economic hedging activities	59	(33)	41	28
Reversal of previously recognized unrealized (gains)/losses on settled positions related to trading activity	(4)	2	(19)	10
Net unrealized gains on open positions related to trading activity	16	11	17	22
Total unrealized mark-to-market gains/(losses) for trading activity	12	13	(2)	32
Total unrealized losses	71	(20)	39	60
Credit Risk Related Contingent Features
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty	6		6
Adequate Assurance Clauses
Credit Risk Related Contingent Features
Derivative net liability position, collateral required Contracts with Credit Rating Contingent Feature	36		36
Credit Rating Contingent Features
Credit Risk Related Contingent Features
Derivative net liability position, collateral required Contracts with Credit Rating Contingent Feature	39		39
Commodity contracts
Unrealized mark-to-market results
Total unrealized losses	71	(20)	39	60
Commodity contracts | Sales
Unrealized mark-to-market results
Total unrealized losses	53	(460)	157	(390)
Commodity contracts | Cost of Sales
Unrealized mark-to-market results
Total unrealized losses	18	440	(118)	450
Interest rate contracts
Unrealized mark-to-market results
Total unrealized losses	$ (24)	$ (7)	$ (19)	$ (18)