Debt - Narrative (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2024 USD ($)	Jun. 30, 2024 USD ($) 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		1,200
Collateral, at fair value		$ 1,200
Bank Term Funding Program Advance
Debt Instrument [Line Items]
Proceeds from short-term debt	$ 100
Weighted average interest rate (in percent)		4.76%
ACST III | London Interbank Offered Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		2.00%
ACST III | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		2.00%
Junior Subordinated Debt, Preferred Securities | ACST II | London Interbank Offered Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		3.15%
Junior Subordinated Debt, Preferred Securities | ACST II | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		3.15%
Federal Home Loan Bank of New York
Debt Instrument [Line Items]
Collateral, at carrying cost		$ 861
Collateral, at fair value		$ 717