Funding Sources (Details) - USD ($)	1 Months Ended				3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 30, 2016	Nov. 30, 2015	Oct. 31, 2015	Sep. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Mar. 31, 2013	Mar. 31, 2016	Jul. 10, 2013	Mar. 31, 2016	Nov. 19, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfers of financial assets accounted for as a sale, initial fair value of assets obtained as proceeds				$ 1,400,000,000			$ 1,200,000,000
Maximum funding limit		$ 4,300,000,000
Receivables Facility, maximum borrowing capacity					$ 2,000,000,000				$ 2,000,000,000		$ 2,000,000,000
Receivables Facility, Amount of Funds Available					94,000,000				94,000,000		94,000,000
Trading Securities, Other					1,155,000,000		1,198,000,000		$ 1,155,000,000		$ 1,155,000,000
Proceeds from sale-leaseback transaction					$ 600,000,000
Receivables Facility, maximum amount available based on receivables sold to investors, percent					50.00%				50.00%		50.00%
Derecognized assets, gross				1,900,000,000
Cash received for sale of receivables under receivables facility			$ 300,000,000	400,000,000	$ 125,000,000
Receivable from Conduits				$ 1,500,000,000	116,000,000		0		$ 116,000,000		$ 116,000,000
Sale Leaseback Transaction, Net Book Value					1,200,000,000	$ 1,300,000,000			1,200,000,000		1,200,000,000	$ 1,300,000,000
Proceeds from sale-leaseback transaction						1,100,000,000		$ 0		$ 0
Receivables from sale-leaseback						126,000,000
Gain (Loss) on Disposition of Property Plant Equipment						$ 65,000,000
Sale Leaseback Transaction, Monthly Rental Payments											277,000,000
Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables Facility, Amount of Funds Available					43,000,000				43,000,000		43,000,000
Trading Securities, Other					760,000,000				760,000,000		760,000,000
Derecognized assets, gross							1,800,000,000
Cash received for sale of receivables under receivables facility							500,000,000
Receivable from Conduits							$ 1,300,000,000
Cash Flows Between Transferor and Transferee, Proceeds from Collections Reinvested in Revolving Period Transfers									207,000,000
Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfers of financial assets accounted for as a sale, initial fair value of assets obtained as proceeds			1,000,000,000
Receivables Facility, Amount of Funds Available					0				0		0
Trading Securities, Other					395,000,000				395,000,000		395,000,000
Derecognized assets, gross			1,200,000,000
Cash received for sale of receivables under receivables facility		$ 400,000,000	100,000,000
Receivable from Conduits			$ 1,100,000,000
Cash Flows Between Transferor and Transferee, Proceeds from Collections Reinvested in Revolving Period Transfers									227,000,000
Subsequent Event [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Bridge Loan	$ 2,000,000,000
Network Sale-Leaseback [Member] | Subsequent Event [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale-leaseback transaction	2,200,000,000
Sale Leaseback Transaction, Net Book Value	$ 3,000,000,000
Future lease receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables Facility, Amount of Funds Available					51,000,000				51,000,000		51,000,000
Revolving Credit Facility [Member] | Unsecured Revolving Credit Facility expires in 2018 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity					$ 3,000,000,000				$ 3,000,000,000		$ 3,000,000,000