Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 5,926,785	$ 6,397,658
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	5,409,698	6,587,581
Assets held by insurance regulatory requirements	11,800	13,400
Assets held in trust for the benefit of contractual counterparties	128,400	189,800
Collateral pledged for FHLB advance		167,200
Fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	5,926,785	6,397,658
Gross Unrealized Gains	5,953	218,390
Gross Unrealized Losses	(523,040)	(28,467)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	5,409,698	6,587,581
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	145,581	133,990
Gross Unrealized Gains	2	285
Gross Unrealized Losses	(9,683)	(868)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	135,900	133,407
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,400,261	2,408,688
Gross Unrealized Gains	4,866	133,361
Gross Unrealized Losses	(256,073)	(7,396)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	2,149,054	2,534,653
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,416,475	2,704,586
Gross Unrealized Gains	1,043	75,172
Gross Unrealized Losses	(196,377)	(13,776)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	2,221,141	2,765,982
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	126,723	150,888
Gross Unrealized Gains	5	830
Gross Unrealized Losses	(6,041)	(1,008)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	120,687	150,710
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	223,743	309,991
Gross Unrealized Gains	10	2,397
Gross Unrealized Losses	(25,744)	(3,278)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	198,009	309,110
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	257,785	315,330
Gross Unrealized Gains	22	5,736
Gross Unrealized Losses	(20,591)	(1,936)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	237,216	319,130
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	337,656	360,436
Gross Unrealized Gains	5	609
Gross Unrealized Losses	(7,829)	(106)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	329,832	360,939
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Total at end of period	4,486	13,749
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(699)	(99)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	3,787	$ 13,650
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Total at end of period	14,075
Gross Unrealized Gains	0
Gross Unrealized Losses	(3)
Fixed income, available-for-sale, at fair value (amortized cost, 2022 - $5,926,785; 2021 - $6,397,658)	$ 14,072

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