Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) Contract	Dec. 31, 2020 USD ($) Contract	Jan. 31, 2021 USD ($)
Guarantor Obligations [Line Items]
Contracts outstanding \| Contract	11,877	10,997
Remaining weighted average contract period outstanding (in years)	10 years 8 months 12 days	10 years 8 months 12 days
Principal	$ 83,829.0	$ 75,287.7	$ 800.0
Interest	39,528.4	36,448.8
Total debt service outstanding	123,357.4	111,736.5
Total gross unearned insurance premiums	$ 257.0	$ 237.5

Remaining weighted average contract period of financial guarantee insurance contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 55
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=99379334&loc=SL5751133-161288

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 9
-Subparagraph (a)(5)(ii)
-URI http://asc.fasb.org/extlink&oid=99382878&loc=SL5749324-161292

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