Debt - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) class subsidiaryBusinessTrust bank	Dec. 31, 2022 USD ($)
Debt Instrument [Line Items]
Number of banks | bank	2
Number of classes of financial instruments | class	2
Number of business subsidiary trusts | subsidiaryBusinessTrust	2
Federal Reserve Bank of New York
Debt Instrument [Line Items]
Collateral, at carrying cost	$ 1,000	$ 1,000
Federal Home Loan Bank of New York
Debt Instrument [Line Items]
Collateral, at carrying cost	865	$ 703
Interest Rate Swap
Debt Instrument [Line Items]
Amount of hedged item	$ 20
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%