Debt - Additional Information (Details)			9 Months Ended
	Jul. 01, 2025 USD ($)	Mar. 29, 2024 USD ($) extension_option	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)
Debt Instrument [Line Items]
Activity-based common stock ownership percentage			0.041
FHLB stock			$ 33,700,000	$ 34,500,000
Short-term debt, face amount			396,000,000
Estimate of Fair Value Measurement | Asset Pledged as Collateral
Debt Instrument [Line Items]
Fair value of pledged assets			1,300,000,000
FHLB borrowings
Debt Instrument [Line Items]
Short-term debt, face amount			65,000,000		$ 17,000,000
Private Placement
Debt Instrument [Line Items]
Purchase price of sale of securities	$ 500,000,000
Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 1,000,000,000	1,000,000,000
Potential maximum borrowing capacity			$ 1,250,000,000
Number of extension options | extension_option		2
Extension term		1 year
Junior Subordinated Debentures Due 2056
Debt Instrument [Line Items]
Interest rate			4.25%
Senior Notes
Debt Instrument [Line Items]
Optional redemption, period from issuance date			5 years
Senior Notes | Senior Notes Due 2055
Debt Instrument [Line Items]
Interest rate	6.58%
Facility agreement term	30 years
Amount of debt	$ 500,000,000
Semi-annual facility fee	1.789%
FHLB borrowings
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 648,000,000
Long-term line of credit			$ 427,000,000	$ 372,000,000