Loans and Leases - Loan Modification Subsequently Defaulted (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	$ 16	$ 6	$ 18	$ 11
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	12	5	13	9
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	0	0	0
Combination - Interest Rate Reduction and Term/Payment Extension/Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	4	1	5	2
Loans and leases held for investment: \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6		6	4
Loans and leases held for investment: \| Commercial real estate \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6		6	4
Loans and leases held for investment: \| Commercial real estate \| Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0		0	0
Loans and leases held for investment: \| Commercial real estate \| Combination - Interest Rate Reduction and Term/Payment Extension/Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0		0	0
Loans and leases held for investment: \| One-to-four family first mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	10	2	12	1
Loans and leases held for investment: \| One-to-four family first mortgage \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	6	1	7	1
Loans and leases held for investment: \| One-to-four family first mortgage \| Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	0	0	0	0
Loans and leases held for investment: \| One-to-four family first mortgage \| Combination - Interest Rate Reduction and Term/Payment Extension/Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total	$ 4	1	$ 5	0
Loans and leases held for investment: \| Commercial and Industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		4		5
Loans and leases held for investment: \| Commercial and Industrial \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		4		4
Loans and leases held for investment: \| Commercial and Industrial \| Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		0		0
Loans and leases held for investment: \| Commercial and Industrial \| Combination - Interest Rate Reduction and Term/Payment Extension/Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total		$ 0		1
Loans and leases held for investment: \| Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total				1
Loans and leases held for investment: \| Other \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total				0
Loans and leases held for investment: \| Other \| Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total				0
Loans and leases held for investment: \| Other \| Combination - Interest Rate Reduction and Term/Payment Extension/Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total				$ 1

FLAGSTAR BANK, NATIONAL ASSOCIATION 10-Q