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Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
--- | --- | ---
Carrying amount and estimated fair value financial instruments |  |
Other assets | $ 132 | $ 132
Repurchase agreement liability | 36 | 37
Carrying Amount |  |
Carrying amount and estimated fair value financial instruments |  |
Other assets | 98 | 134
Financial guaranty insurance contracts | (2,422) | (2,394)
Long-term debt | (1,673) | (1,673)
Other liabilities | (61) | (45)
Carrying Amount | Consolidated Investment Vehicles |  |
Carrying amount and estimated fair value financial instruments |  |
Assets of CIVs | 176 | 171
Liabilities of CIVs | (859) | (586)
Estimated Fair Value |  |
Carrying amount and estimated fair value financial instruments |  |
Other assets | 99 | 135
Financial guaranty insurance contracts | (1,677) | (2,315)
Long-term debt | (1,677) | (1,832)
Other liabilities | (61) | (45)
Estimated Fair Value | Consolidated Investment Vehicles |  |
Carrying amount and estimated fair value financial instruments |  |
Assets of CIVs | 176 | 171
Liabilities of CIVs | $ (859) | $ (586)

X
- Definition This element represents the net financial guaranty contract position for the entity, consisting of premiums receivable, ceded unearned premium reserve, reinsurance recoverable on unpaid losses, salvage and subrogation recoverable, unearned premium reserve, loss and loss adjustment expense reserve, and reinsurance balances payable, net. + References No definition available. + Details
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Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(m)(1)(ii)(A)(ii)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(m)(1)(i)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 + Details
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Balance Type: | credit
Period Type: | instant

X
- Definition Amount receivable from correspondent broker-dealer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=123384075&loc=d3e41242-110953 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=35710130&loc=d3e41645-110958 + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Amount payable to correspondent broker-dealer. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Balance Type: | na
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Fair value portion of other assets. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Fair value portion of other liabilities. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Details
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- Details
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Namespace Prefix: |
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- Details
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