Installment Receivables - Schedule of Activity in the Deferred Interest Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Deferred interest and allowance for credit losses, beginning of period	$ 323	$ 506
Bad debt expense	66	142
Write-offs, net of recoveries	(88)	(224)
Change in deferred interest on short- and long-term installment receivables	6	(101)
Deferred interest and allowance for credit losses, end of period	257	323
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	$ (50)	$ 0