Fair Value Measurements, Recurring (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Investments of certain benefit plans	$ 72	$ 73
Total gross fair value, assets	2,156	967
Effect of counterparty netting	(2,058)	(893)
Effect of cash collateral netting	0	0
Net carrying value on Balance Sheet, assets	98	74
Liabilities:
Environmental credit obligations	16	104
Total gross fair value, liabilities	2,149	1,086
Effect of counterparty netting	(2,058)	(893)
Effect of cash collateral netting	(61)	(76)
Net carrying value on Balance Sheet, liabilities	30	117
Commodity Contracts [Member]
Assets:
Derivative contracts	2,084	894
Effect of counterparty netting	(2,058)	(893)
Effect of cash collateral netting	0	0
Derivative contracts, net assets	26	1
Cash collateral received not offset	0	0
Liabilities:
Derivative contracts	2,119	969
Effect of counterparty netting	(2,058)	(893)
Effect of cash collateral netting	(61)	(76)
Derivative contracts, net liabilities	0	0
Cash collateral paid not offset	(88)	(102)
Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	11	6
Derivative contracts, net liabilities	11	6
Foreign Currency Contracts [Member]
Liabilities:
Derivative contracts	3	7
Derivative contracts, net liabilities	3	7
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	63	65
Total gross fair value, assets	2,135	940
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	2,112	962
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	2,072	875
Liabilities:
Derivative contracts	2,109	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]
Liabilities:
Derivative contracts	3	7
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	12	19
Liabilities:
Environmental credit obligations	16	104
Total gross fair value, liabilities	37	124
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	12	19
Liabilities:
Derivative contracts	10	14
Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	11	6
Fair Value, Inputs, Level 2 [Member] | Foreign Currency Contracts [Member]
Liabilities:
Derivative contracts	0	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]
Liabilities:
Derivative contracts	$ 0	$ 0