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Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 68,511	$ 68,185	$ 34,444	$ 55,967	$ 57,594	$ 55,826	$ 58,505	$ 50,618	$ 227,107		$ 222,543		$ 195,336
Investments Accounted For Under The Equity Method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income										$ (26,000)		$ (8,000)		$ 134,000
Realized gains										1,452,000		695,000		1,981,000
Net change in unrealized appreciation										4,898,000		5,543,000		1,303,000
Net Income before tax										6,324,000		6,230,000		$ 3,418,000
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned										$ 26,500		$ 17,900			$ 17,600

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

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