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Investments -Schedule of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income [Line Items]
Net investment income earned	$ 117,759	$ 100,863	$ 334,250	$ 290,065
Fixed income securities
Net Investment Income [Line Items]
Net investment income earned	98,464	90,013	286,501	254,016
Commercial mortgage loans ("CMLs")
Net Investment Income [Line Items]
Net investment income earned	3,238	2,516	9,177	6,680
Equity securities
Net Investment Income [Line Items]
Net investment income earned	5,362	2,083	12,147	5,524
Short-term investments
Net Investment Income [Line Items]
Net investment income earned	6,457	3,941	14,656	11,483
Alternative investments
Net Investment Income [Line Items]
Net investment income earned	9,031	6,473	26,429	25,637
Other investments
Net Investment Income [Line Items]
Net investment income earned	251	284	632	515
Investment expenses
Net Investment Income [Line Items]
Investment expenses	$ (5,044)	$ (4,447)	$ (15,292)	$ (13,790)

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