Loans and Leases - Performance of Loans Modified in Last 12 Months (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	$ 17	$ 35
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6	25
30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	2
90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	11	8
Loans and Leases Held for Investment: | Multi-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		8
Loans and Leases Held for Investment: | Multi-family | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		8
Loans and Leases Held for Investment: | Multi-family | 30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		0
Loans and Leases Held for Investment: | Multi-family | 90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		0
Loans and Leases Held for Investment: | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	4	2
Loans and Leases Held for Investment: | Commercial real estate | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	2
Loans and Leases Held for Investment: | Commercial real estate | 30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	0
Loans and Leases Held for Investment: | Commercial real estate | 90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	4	0
Loans and Leases Held for Investment: | One-to-four family first mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	12	10
Loans and Leases Held for Investment: | One-to-four family first mortgage | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6	2
Loans and Leases Held for Investment: | One-to-four family first mortgage | 30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	0
Loans and Leases Held for Investment: | One-to-four family first mortgage | 90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6	8
Other loans held for investment | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	1	14
Other loans held for investment | Commercial and industrial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	12
Other loans held for investment | Commercial and industrial | 30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	2
Other loans held for investment | Commercial and industrial | 90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	$ 1	0
Other loans held for investment | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		1
Other loans held for investment | Other | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		1
Other loans held for investment | Other | 30 - 89 Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		0
Other loans held for investment | Other | 90+ Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		$ 0