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Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis Using Significant Unobservable Inputs (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value $ 127 $ 17 $ 94 $ 49
Total gains (losses) included in net income 0 23 18 5
Total gains (losses) included in other comprehensive income (1) 0 1 0
Purchases 10 16 49 41
Issuances 0 0 0 0
Sales 0 0 0 (4)
Settlements (5) 3 (6) (32)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 (25) 0
Fair value 131 59 131 59
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 23 18 5
Available-for-sale debt securities [Member]        
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value 127 76 93 70
Total gains (losses) included in net income 0 0 0 0
Total gains (losses) included in other comprehensive income (1) 0 1 0
Purchases 10 16 49 41
Issuances 0 0 0 0
Sales 0 0 0 (4)
Settlements (5) (3) (12) (18)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 131 89 131 89
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0 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 46 30 40 31
Total gains (losses) included in net income 0 0 0 0
Total gains (losses) included in other comprehensive income (1) 0 0 0
Purchases 6 11 11 16
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements (1) 0 (1) (6)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 50 41 50 41
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0 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 81 46 53 39
Total gains (losses) included in net income 0 0 0 0
Total gains (losses) included in other comprehensive income 0 0 1 0
Purchases 4 5 38 25
Issuances 0 0 0 0
Sales 0 0 0 (4)
Settlements (4) (3) (11) (12)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 81 48 81 48
The amount of change in unrealized gains (losses) included in net income attributable to assets held at the reporting date 0 0 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 0 23 18 5
Fair value 0 (59) 1 (21)
Total gains (losses) included in net income 0 23 18 5
Total gains (losses) included in other comprehensive income 0 0 0 0
Purchases 0 0 0 0
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements 0 6 6 (14)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 (25) 0
Fair value $ 0 $ (30) $ 0 $ (30)