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Investments (Pre-tax Net Investment Income Earned) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Statement [Line Items]
Net investment income earned	$ 388,650		$ 288,155		$ 326,589
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned	345,886		259,918		209,709
Commercial Mortgage Loans [Member]
Statement [Line Items]
Net investment income earned	9,336		5,555		2,743
Equity Securities [Member]
Statement [Line Items]
Net investment income earned	9,395		13,554		15,920
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned	14,818		3,997		260
Alternative Investments [Member]
Statement [Line Items]
Net investment income earned	26,777	$ 26,800	23,003	$ 23,000	117,701	$ 117,700
Other Investment [Member]
Statement [Line Items]
Net investment income earned	650		258		359
Investment Income (Expense)
Statement [Line Items]
Investment Expense	$ (18,212)		$ (18,130)		$ (20,103)

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Amount of expenses related to the generation of investment income.

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