Investments - Schedule of Composition of Fair Value of Debt Securities Classified by Rating (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Summary of Investment Holdings [Line Items]
Debt securities, rated by investment advisor	$ 848	$ 606
Percent of Total Debt Securities Carrying Value	100.00%	100.00%
Investment grade
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	94.70%	93.50%
Investment grade \| U.S. government securities
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	7.80%	5.90%
Investment grade \| AAA
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	4.70%	5.00%
Investment grade \| AA
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	9.20%	9.50%
Investment grade \| A
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	32.20%	32.20%
Investment grade \| BBB
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	40.80%	40.90%
Below investment grade
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	5.30%	6.50%
Below investment grade \| BB
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	1.90%	2.50%
Below investment grade \| B and below
Summary of Investment Holdings [Line Items]
Percent of Total Debt Securities Carrying Value	3.40%	4.00%