Debt - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 07, 2024 USD ($)	Sep. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2024 USD ($) class subsidiaryBusinessTrust
Debt Instrument [Line Items]
Repayments of short-term debt		$ 5
Number of classes of financial instruments | class				2
Number of business subsidiary trusts | subsidiaryBusinessTrust				2
Subsequent Event
Debt Instrument [Line Items]
Repayments of short-term debt	$ 95
Interest Rate Swap
Debt Instrument [Line Items]
Amount of hedged item		20		$ 20
Federal Reserve Bank Advances
Debt Instrument [Line Items]
Collateral, at carrying cost		1,100		1,100
Collateral, at fair value		$ 1,100		$ 1,100
Bank Term Funding Program Advance
Debt Instrument [Line Items]
Proceeds from short-term debt			$ 100
Weighted average interest rate (in percent)		4.76%		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		$ 890		$ 890
Collateral, at fair value		$ 755		$ 755