Investments - Schedule of Securities Pledged by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	$ 172	$ 157
Total gross secured borrowings	379	157
Repurchase Agreements
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	207
Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	161	144
U.S. government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements		1
Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	8
Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	3	12
Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total gross secured borrowings	172	157
Overnight and Continuous | Repurchase Agreements
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
Overnight and Continuous | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	161	144
Overnight and Continuous | U.S. government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements		1
Overnight and Continuous | Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	8
Overnight and Continuous | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	3	12
Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total gross secured borrowings	0	0
Up to 30 Days | Repurchase Agreements
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
Up to 30 Days | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0	0
Up to 30 Days | U.S. government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements		0
Up to 30 Days | Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
Up to 30 Days | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0	0
30-90 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total gross secured borrowings	207	0
30-90 Days | Repurchase Agreements
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	207
30-90 Days | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0	0
30-90 Days | U.S. government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements		0
30-90 Days | Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
30-90 Days | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0	0
Greater Than 90 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total gross secured borrowings	0	0
Greater Than 90 Days | Repurchase Agreements
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
Greater Than 90 Days | Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0	0
Greater Than 90 Days | U.S. government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements		0
Greater Than 90 Days | Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	0
Greater Than 90 Days | Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities pledged under securities lending agreements	$ 0	$ 0