Fair Value Measurements, Recurring (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Assets:
Investments of certain benefit plans	$ 70	$ 69
Total gross fair value, assets	743	984
Effect of Counterparty Netting	(637)	(875)
Effect of Cash Collateral Netting	0	0
Net Carrying Value on Balance Sheet, Assets	106	109
Liabilities:
Environmental credit obligations	289	188
Total gross fair value, liabilities	939	1,088
Effect of Counterparty Netting	(637)	(875)
Effect of Cash Collateral Netting	(8)	(20)
Net Carrying Value on Balance Sheet, Liabilities	294	193
Commodity Contracts [Member]
Assets:
Derivative contracts	673	912
Effect of Counterparty Netting	(637)	(875)
Effect of Cash Collateral Netting	0	0
Derivative contracts, net assets	36	37
Cash Collateral Received Not Offset	0	0
Liabilities:
Derivative contracts	645	895
Effect of Counterparty Netting	(637)	(875)
Effect of Cash Collateral Netting	(8)	(20)
Derivative contracts, net liabilities	0	0
Cash Collateral Paid Not Offset	(61)	(88)
Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	3	5
Derivative contracts, net liabilities	3	5
Foreign Currency Contracts [Member]
Assets:
Derivative contracts		3
Derivative contracts, net assets		3
Liabilities:
Derivative contracts	2
Derivative contracts, net liabilities	2
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	59	58
Total gross fair value, assets	720	935
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	638	872
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	661	874
Liabilities:
Derivative contracts	636	872
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		3
Liabilities:
Derivative contracts	2
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	12	38
Liabilities:
Environmental credit obligations	289	188
Total gross fair value, liabilities	301	216
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	12	38
Liabilities:
Derivative contracts	9	23
Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	3	5
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	11	11
Total gross fair value, assets	11	11
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