Investments - Composition of Fair Value of Debt Securities Classified by Rating (Details)	Sep. 30, 2023	Dec. 31, 2022
Summary of Investment Holdings [Line Items]
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	92.50%	92.40%
Investment grade | U.S. government securities
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	8.90%	11.60%
Investment grade | AAA
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	6.60%	6.40%
Investment grade | AA
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	9.10%	8.20%
Investment grade | A
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	31.40%	29.80%
Investment grade | BBB
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	36.50%	36.40%
Below investment grade
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	7.50%	7.60%
Below investment grade | BB
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	3.80%	3.90%
Below investment grade | B and below
Summary of Investment Holdings [Line Items]
Percent of total debt securities carrying value	3.70%	3.70%