Installment Receivables - Schedule of Activity in the Deferred Interest Allowance for Credit Losses (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Deferred interest and allowance for credit losses, beginning of period	$ 286	$ 323
Bad debt expense	90	116
Write-offs, net of recoveries	(72)	(118)
Change in deferred interest on short- and long-term installment receivables	6	15
Deferred interest and allowance for credit losses, end of period	310	286
Balances without adoption of Topic 606
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Change in deferred interest on short- and long-term installment receivables	$ 0	$ (50)