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Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Net investment income earned	$ 100,863	$ 63,889	$ 290,065	$ 206,713
Fixed Income Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	90,013	68,236	254,016	184,305
Commercial Mortgage Loans [Member]
Net Investment Income [Line Items]
Net investment income earned	2,516	1,600	6,680	3,762
Equity Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	2,083	2,604	5,524	7,661
Short-Term Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	3,941	1,152	11,483	1,660
Alternative Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	6,473	(5,581)	25,637	22,821
Other Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	284	112	515	75
Investment Income (Expense)
Net Investment Income [Line Items]
Investment Expense	$ (4,447)	$ (4,234)	$ (13,790)	$ (13,571)

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Amount of expenses related to the generation of investment income.

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