Derivative Assets and Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value
Gross derivative assets		$ 813	$ 839
Gross derivative liabilities		874	888
Derivative Asset, Collateral, Obligation to Return Cash, Offset	[1],[2]	661	744
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	[1],[2]	781	832
Net derivative assets		152	95
Net derivative liabilities		93	56
Derivative, Collateral
Cash collateral outstanding		120
Additional Collateral, Aggregate Fair Value			88
Commodity Futures Contracts
Derivatives, Fair Value
Derivative Asset, Collateral, Obligation to Return Cash, Offset	[2]	637	733
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	[2]	757	821
Net derivative assets		126	88
Net derivative liabilities		69	50
Commodity Futures Contracts | Prepayments and other current assets
Derivatives, Fair Value
Gross derivative assets		763	821
Gross derivative liabilities		826	871
Commodity Swap Contracts
Derivatives, Fair Value
Derivative Asset, Collateral, Obligation to Return Cash, Offset	[2]	24	11
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	[2]	24	11
Net derivative assets		10	0
Net derivative liabilities		15	4
Commodity Swap Contracts | Prepayments and other current assets
Derivatives, Fair Value
Gross derivative assets		31	11
Gross derivative liabilities		34	13
Commodity Swap Contracts | Accounts Receivable [Member]
Derivatives, Fair Value
Gross derivative assets		3	0
Gross derivative liabilities		0	0
Commodity Swap Contracts | Accounts payable
Derivatives, Fair Value
Gross derivative assets		0	0
Gross derivative liabilities		5	2
Commodity Options Contracts
Derivatives, Fair Value
Derivative Asset, Collateral, Obligation to Return Cash, Offset	[2]	0	0
Net derivative assets		3	1
Commodity Options Contracts | Prepayments and other current assets
Derivatives, Fair Value
Gross derivative assets		3	1
Gross derivative liabilities		1	0
Commodity Forward Contracts
Derivatives, Fair Value
Derivative Asset, Collateral, Obligation to Return Cash, Offset	[2]	0	0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	[2]	0	0
Net derivative assets		13	6
Net derivative liabilities		8	2
Commodity Forward Contracts | Accounts Receivable [Member]
Derivatives, Fair Value
Gross derivative assets		13	6
Gross derivative liabilities		0	0
Commodity Forward Contracts | Accounts payable
Derivatives, Fair Value
Gross derivative assets		0	0
Gross derivative liabilities		$ 8	$ 2

[1] Certain of our derivative contracts, under master netting arrangements, include both asset and liability positions. We offset both the fair value amounts and any related cash collateral amounts recognized for multiple derivative instruments executed with the same counterparty when there is a legally enforceable right and an intention to settle net or simultaneously. As of September 30, 2017 and December 31, 2016, we had provided cash collateral amounts of $120 million and $88 million, respectively, related to our unrealized derivative positions. Cash collateral amounts are netted with mark-to-market derivative assets.

[2] Certain of our derivative contracts, under master netting arrangements, include both asset and liability positions. We offset both the fair value amounts and any related cash collateral amounts recognized for multiple derivative instruments executed with the same counterparty when there is a legally enforceable right and an intention to settle net or simultaneously. As of September 30, 2017 and December 31, 2016, we had provided cash collateral amounts of $120 million and $88 million, respectively, related to our unrealized derivative positions. Cash collateral amounts are netted with mark-to-market derivative assets.