Investment Securities - Pledged Securities and Concentration Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2022
Concentration of risk
Credit losses recognized on securities	$ 0	$ 0	$ 0
Asset Pledged as Collateral
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investment securities pledged as collateral	6,300,000,000	6,300,000,000	3,200,000,000
Asset Pledged as Collateral | Public deposits
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investment securities pledged as collateral	3,500,000,000	3,500,000,000	3,000,000,000.0
Asset Pledged as Collateral | Other financial transactions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investment securities pledged as collateral	180,100,000	180,100,000	$ 207,800,000
Borrowing Capacity
Financial Instruments Owned and Pledged as Collateral [Line Items]
Investment securities pledged as collateral	2,600,000,000	2,600,000,000
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0	$ 0