Investment Securities - Narrative (Details) - USD ($)	3 Months Ended						12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025	Sep. 30, 2025	Mar. 31, 2025	Jun. 30, 2022	Sep. 30, 2021	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Debt securities, reclassified			$ 3,590,000,000
Proceeds from sale of available-for-sale securities		$ 3,160,000,000
Pre-tax loss		$ 625,600,000
Income tax benefit related to security gains (losses) (as percent)		21.946%					21.946%
Debt securities, available-for-sale, transfer to held-to-maturity					$ 1,990,000,000	$ 500,800,000
Available-for-sale, transfer to held-to-maturity, gain (loss)	$ 0
Available for sale mortgage backed securities	3,547,987,000	$ 3,642,809,000					$ 3,642,809,000
Available-for-sale, interest receivable	20,800,000
Available-for-sale securities	3,152,286,000	3,266,221,000					3,266,221,000
Securities valued at less than historical cost, amount	$ 2,604,076,000	2,660,240,000					2,660,240,000
Securities valued at less than historical cost (as percent)	82.60%
Provision for credit losses, AFS securities	$ 0			$ 0
Securities pledged as collateral	1,850,000,000	2,040,000,000.00					2,040,000,000.00
Realized gains	0			0
Realized losses	0			$ 0
Variable rate commercial MBS
Debt Securities, Available-for-sale [Line Items]
Available for sale mortgage backed securities	575,700,000	597,400,000					597,400,000
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Available for sale mortgage backed securities	$ 1,550,000,000	$ 1,600,000,000					$ 1,600,000,000