Investments - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($) issuer	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) security issuer	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Gain (Loss) on Securities [Line Items]
Fixed maturity securities, available-for-sale	$ 1,875,695,000		$ 1,875,695,000		$ 1,783,417,000
Bank loan participations	143,762,000		143,762,000		154,991,000
Unpaid principal balance	156,300,000		156,300,000
Net investment income	25,175,000	$ 14,705,000	50,947,000	$ 30,972,000
Net gain (loss) in bank loan participations	2,116,000	(9,895,000)	3,531,000	(11,993,000)
Unrealized losses	497,000	0	2,400,000	0
Bank Loan Participations
Gain (Loss) on Securities [Line Items]
Net investment income	$ 4,400,000	$ 2,700,000	$ 8,700,000	$ 5,100,000
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Number of issuers | issuer	552		552
Total fair value	$ 1,707,355,000		$ 1,707,355,000		1,588,192,000
Gross unrealized losses	173,776,000		173,776,000		189,491,000
Fixed maturity securities, available-for-sale	1,875,695,000		1,875,695,000		1,783,417,000
Aggregate net unrealized loss	$ 173,776,000		173,776,000		$ 189,491,000
Impairment losses			$ 85,000
Number of impaired securities | security			1
Fixed maturity securities | BBB- or better
Gain (Loss) on Securities [Line Items]
Percentage of available for sale securities	99.90%		99.90%
Fixed maturity securities | Below BBB-
Gain (Loss) on Securities [Line Items]
Fixed maturity securities, available-for-sale	$ 2,300,000		$ 2,300,000
Aggregate net unrealized loss	$ 160,000		$ 160,000