Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis Using Significant Unobservable Inputs (Details) - Available-for-sale debt securities [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	$ 105	$ 127	$ 118	$ 93
Total gains (losses) included in other comprehensive income	1	(1)	9	1
Purchases	0	10	2	49
Issuances	0	0	0	0
Sales	0	0	(10)	0
Settlements	(3)	(5)	(16)	(12)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value	103	131	103	131
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date	2		9
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	47	46	46	40
Total gains (losses) included in other comprehensive income	1	(1)	2	0
Purchases	0	6	0	11
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	(1)	0	(1)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value	48	50	48	50
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date	1		2
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	58	81	72	53
Total gains (losses) included in other comprehensive income	0	0	7	1
Purchases	0	4	2	38
Issuances	0	0	0	0
Sales	0	0	(10)	0
Settlements	(3)	(4)	(16)	(11)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value	55	$ 81	55	$ 81
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date	$ 1		$ 7