Debt - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) class subsidiaryBusinessTrust
Debt Instrument [Line Items]
Number of classes of financial instruments | class	2
Number of business subsidiary trusts | subsidiaryBusinessTrust	2
Interest Rate Swap
Debt Instrument [Line Items]
Amount of hedged item	$ 20
Federal Reserve Bank Advances
Debt Instrument [Line Items]
Collateral, at carrying cost	1,100
Collateral, at fair value	1,100
Bank Term Funding Program Advance
Debt Instrument [Line Items]
Proceeds from short-term debt	$ 100
Weighted average interest rate	4.76%
ACST III | London Interbank Offered Rate
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
ACST III | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Junior Subordinated Debt, Preferred Securities | ACST II | London Interbank Offered Rate
Debt Instrument [Line Items]
Basis spread on variable rate	3.15%
Junior Subordinated Debt, Preferred Securities | ACST II | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on variable rate	3.15%
Federal Home Loan Bank of New York
Debt Instrument [Line Items]
Collateral, at carrying cost	$ 875
Collateral, at fair value	$ 737