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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, 2018
Mar. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Total gains (losses) included in earnings $ 7 $ (67)
Total gains (losses) included in other comprehensive income 0 2
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 (67)
Fair value 13 116
Purchases 35 10
Issuances 0 0
Sales 0 0
Settlements (5) (48)
Transfers in to Level 3 1 0
Transfers out of Level 3 (2) 0
Fair value 49 13
Available-for-sale securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 80 91
Total gains (losses) included in earnings 0 0
Total gains (losses) included in other comprehensive income 0 2
Purchases 35 10
Issuances 0 0
Sales 0 0
Settlements (44) (23)
Transfers in to Level 3 1 0
Transfers out of Level 3 (2) 0
Fair value 70 80
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 0
Available-for-sale securities [Member] | U.S. government and agency obligations [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 2 2
Total gains (losses) included in earnings 0 0
Total gains (losses) included in other comprehensive income 0 0
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 0 0
Transfers in to Level 3 0 0
Transfers out of Level 3 (2) 0
Fair value 0 2
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 0
Available-for-sale securities [Member] | Corporate debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 8 7
Total gains (losses) included in earnings 0 0
Total gains (losses) included in other comprehensive income 0 1
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements (9) 0
Transfers in to Level 3 1 0
Transfers out of Level 3 0 0
Fair value 0 8
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 0
Available-for-sale securities [Member] | Mortgage-backed securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 39 45
Total gains (losses) included in earnings 0 0
Total gains (losses) included in other comprehensive income 0 0
Purchases 5 4
Issuances 0 0
Sales 0 0
Settlements (13) (10)
Transfers in to Level 3 0 0
Transfers out of Level 3 0 0
Fair value 31 39
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 0
Available-for-sale securities [Member] | Asset-backed securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value 31 37
Total gains (losses) included in earnings 0 0
Total gains (losses) included in other comprehensive income 0 1
Purchases 30 6
Issuances 0 0
Sales 0 0
Settlements (22) (13)
Transfers in to Level 3 0 0
Transfers out of Level 3 0 0
Fair value 39 31
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 0
Derivative [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 (67)
Fair value (67) 25
Total gains (losses) included in earnings 7 (67)
Total gains (losses) included in other comprehensive income 0 0
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 39 (25)
Transfers in to Level 3 0 0
Transfers out of Level 3 0 0
Fair value (21) (67)
Derivative [Member] | Interest rate swaps [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 (24)
Fair value (5) 39
Total gains (losses) included in earnings 0 (24)
Total gains (losses) included in other comprehensive income 0 0
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements (16) (20)
Transfers in to Level 3 0 0
Transfers out of Level 3 0 0
Fair value (21) (5)
Derivative [Member] | Foreign currency swaps [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
The amount of total gains (losses) included in earnings attributable to assets held at the reporting date 0 (43)
Fair value (62) (14)
Total gains (losses) included in earnings 7 (43)
Total gains (losses) included in other comprehensive income 0 0
Purchases 0 0
Issuances 0 0
Sales 0 0
Settlements 55 (5)
Transfers in to Level 3 0 0
Transfers out of Level 3 0 0
Fair value $ 0 $ (62)