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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 6 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value $ 120 $ 107 $ 118 $ 93
Total gains (losses) included in other comprehensive income 1 0 8 2
Purchases 2 24 2 39
Issuances 0 0 0 0
Sales (9) 0 (10) 0
Settlements (9) (4) (13) (7)
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 105 127 105 127
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date 0   7  
Mortgage-backed securities [Member]        
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value 48 41 46 40
Total gains (losses) included in other comprehensive income (1) 0 1 1
Purchases 0 5 0 5
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements 0 0 0 0
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 47 46 47 46
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date (1)   1  
Asset-backed securities [Member]        
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value 72 66 72 53
Total gains (losses) included in other comprehensive income 2 0 7 1
Purchases 2 19 2 34
Issuances 0 0 0 0
Sales (9) 0 (10) 0
Settlements (9) (4) (13) (7)
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 58 $ 81 58 $ 81
The amount of total unrealized gains (losses) included in other comprehensive income attributable to assets held at the reporting date $ 1   $ 6