Fair Value Measurements, Recurring (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets:
Investments of certain benefit plans	$ 2,097	$ 1,947
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	1,115	1,025
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	982	922
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	0	0
Fair Value, Measurements, Recurring [Member]
Assets:
Investments of certain benefit plans	69	75
Total gross fair value, assets	984	760
Effect of Counter- party Netting	(875)	(557)
Effect of Cash Collateral Netting	0	(12)
Net Carrying Value on Balance Sheet, Assets	109	191
Liabilities:
Environmental credit obligations	188	2
Total gross fair value, liabilities	1,088	565
Effect of Counter- party Netting	(875)	(557)
Effect of Cash Collateral Netting	(20)	0
Net Carrying Value on Balance Sheet, Liabilities	193	8
Fair Value, Measurements, Recurring [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	912	682
Effect of Counter- party Netting	(875)	(557)
Effect of Cash Collateral Netting	0	(12)
Derivative contracts, net assets	37	113
Cash Collateral Received Not Offset	0	0
Liabilities:
Derivative contracts	895	557
Effect of Counter- party Netting	(875)	(557)
Effect of Cash Collateral Netting	(20)	0
Derivative contracts, net liabilities	0	0
Cash Collateral Paid Not Offset	(88)	(31)
Fair Value, Measurements, Recurring [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	5	6
Derivative contracts, net liabilities	5	6
Fair Value, Measurements, Recurring [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	3	3
Derivative contracts, net assets	3	3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	58	64
Total gross fair value, assets	935	716
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	872	522
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	874	649
Liabilities:
Derivative contracts	872	522
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	3	3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	38	33
Liabilities:
Environmental credit obligations	188	2
Total gross fair value, liabilities	216	43
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	38	33
Liabilities:
Derivative contracts	23	35
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	5	6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	11	11
Total gross fair value, assets	11	11
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	0	0
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	$ 0	$ 0