Fair Value Measurements, Recurring (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Investments of certain benefit plans	$ 70	$ 73
Total gross fair value, assets	1,508	967
Effect of counterparty netting	(1,434)	(893)
Effect of cash collateral netting	0	0
Net carrying value on Balance Sheet, assets	74	74
Liabilities:
Environmental credit obligations	70	104
Total gross fair value, liabilities	1,550	1,086
Effect of counterparty netting	(1,434)	(893)
Effect of cash collateral netting	(32)	(76)
Net carrying value on Balance Sheet, liabilities	84	117
Commodity Contracts [Member]
Assets:
Derivative contracts	1,436	894
Effect of counterparty netting	(1,434)	(893)
Effect of cash collateral netting	0	0
Derivative contracts, net assets	2	1
Cash collateral received not offset	0	0
Liabilities:
Derivative contracts	1,467	969
Effect of counterparty netting	(1,434)	(893)
Effect of cash collateral netting	(32)	(76)
Derivative contracts, net liabilities	1	0
Cash collateral paid not offset	(57)	(102)
Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	13	6
Derivative contracts, net liabilities	13	6
Foreign Currency Contracts [Member]
Assets:
Derivative contracts	2
Derivative contracts, net assets	2
Liabilities:
Derivative contracts		7
Derivative contracts, net liabilities		7
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	61	65
Total gross fair value, assets	1,465	940
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	1,430	962
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	1,402	875
Liabilities:
Derivative contracts	1,430	955
Fair Value, Inputs, Level 1 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 1 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	2
Liabilities:
Derivative contracts		7
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	34	19
Liabilities:
Environmental credit obligations	70	104
Total gross fair value, liabilities	120	124
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	34	19
Liabilities:
Derivative contracts	37	14
Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	13	6
Fair Value, Inputs, Level 2 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	0
Liabilities:
Derivative contracts		0
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	9	8
Total gross fair value, assets	9	8
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	0	0
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	0	0
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 3 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 3 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	$ 0
Liabilities:
Derivative contracts		$ 0