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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 6 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value $ 107 $ 40 $ 94 $ 49
Total gains (losses) included in net income 0 (10) 18 (18)
Total gains (losses) included in other comprehensive income 0 0 2 0
Purchases 24 23 39 25
Issuances 0 0 0 0
Sales 0 (4) 0 (4)
Settlements (4) (32) (1) (35)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 (25) 0
Fair value 127 17 127 17
The amount of total gains (losses) included in net income attributable to assets held at the reporting date 0 (10) 18 (18)
Available-for-sale debt securities [Member]        
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]        
Fair value 107 63 93 70
Total gains (losses) included in net income 0 0 0 0
Total gains (losses) included in other comprehensive income 0 0 2 0
Purchases 24 23 39 25
Issuances 0 0 0 0
Sales 0 (4) 0 (4)
Settlements (4) (6) (7) (15)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 127 76 127 76
The amount of total 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 41 25 40 31
Total gains (losses) included in net income 0 0 0 0
Total gains (losses) included in other comprehensive income 0 0 1 0
Purchases 5 5 5 5
Issuances 0 0 0 0
Sales 0 0 0 0
Settlements 0 0 0 (6)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 46 30 46 30
The amount of total 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 66 38 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 19 18 34 20
Issuances 0 0 0 0
Sales 0 (4) 0 (4)
Settlements (4) (6) (7) (9)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 0 0
Fair value 81 46 81 46
The amount of total 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 total gains (losses) included in net income attributable to assets held at the reporting date 0 (10) 18 (18)
Fair value 0 (23) 1 (21)
Total gains (losses) included in net income 0 (10) 18 (18)
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 (26) 6 (20)
Transfers in to Level 3 0 0 0 0
Transfers out of Level 3 0 0 (25) 0
Fair value $ 0 $ (59) $ 0 $ (59)