Finance Receivables - Summary of Finance Receivables (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Mar. 31, 2014
Accounts Notes And Loans Receivable [Line Items]
Finance receivables	$ 36,201		$ 39,004
Allowance for credit losses	(93)	$ (93)	(86)	$ (89)	$ (94)	$ (100)
Write-down of lease residual values	(11)		(13)
Unearned interest income and fees	(31)		(64)
Deferred dealer participation and IDC	368		393
Unearned subsidy income	(709)		(770)
Finance receivables, net	35,725		38,464	40,203
Lease
Accounts Notes And Loans Receivable [Line Items]
Finance receivables	1,115		1,956
Allowance for credit losses	(2)	(2)	(2)	(3)	(3)	(4)
Write-down of lease residual values	(11)		(13)
Unearned interest income and fees	(31)		(64)
Deferred dealer participation and IDC	2		3
Unearned subsidy income	(38)		(80)
Finance receivables, net	1,035		1,800
Retail
Accounts Notes And Loans Receivable [Line Items]
Finance receivables	30,934		32,792
Allowance for credit losses	(91)	$ (91)	(84)	$ (86)	$ (91)	(95)
Deferred dealer participation and IDC	366		390
Unearned subsidy income	(671)		(690)
Finance receivables, net	30,538		32,408
Dealer
Accounts Notes And Loans Receivable [Line Items]
Finance receivables	4,152		4,256
Allowance for credit losses						$ (1)
Finance receivables, net	$ 4,152		$ 4,256