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Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis Using Significant Unobservable Inputs (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value $ 94 $ 49
Total gains (losses) included in net income 15 29
Total gains (losses) included in other comprehensive income (6) 1
Purchases 48 48
Issuances 0 0
Sales 0 (4)
Settlements (8) (29)
Transfers in to Level 3 0 0
Transfers out of Level 3 (25) 0
Fair value 118 94
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 18 29
Available-for-sale debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 93 70
Total gains (losses) included in net income (3) 0
Total gains (losses) included in other comprehensive income (6) 1
Purchases 48 48
Issuances 0 0
Sales 0 (4)
Settlements (14) (22)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Fair value 118 93
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0
Available-for-sale debt securities [Member] | Mortgage-backed securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 40 31
Total gains (losses) included in net income (1) 0
Total gains (losses) included in other comprehensive income (2) 0
Purchases 10 16
Issuances 0 0
Sales 0 0
Settlements (1) (7)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Fair value 46 40
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0
Available-for-sale debt securities [Member] | Asset-backed securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 53 39
Total gains (losses) included in net income (2) 0
Total gains (losses) included in other comprehensive income (4) 1
Purchases 38 32
Issuances 0 0
Sales 0 (4)
Settlements (13) (15)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Fair value 72 53
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0
Interest rate swaps [Member] | Derivative [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 18 29
Fair value 1 (21)
Total gains (losses) included in net income 18 29
Total gains (losses) included in other comprehensive income 0 0
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 6 (7)
Transfers into Level 3 0 0
Transfers out of Level 3 (25) 0
Fair value $ 0 $ 1