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Derivative Instruments - Summary of Gross Principal Notional Outstanding for CDS Contracts (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Variable Interest Entity [Member] | Interest Rate Swap [Member] |  |
Ambac Rating |  |
Derivative, Notional Amount | $ 1,233 | $ 1,194
Variable Interest Entity [Member] | Interest Rate Swap One [Member] |  |
Ambac Rating |  |
Derivative, Notional Amount | 1,151 | 1,176
Variable Interest Entity [Member] | Currency Swaps [Member] |  |
Ambac Rating |  |
Derivative, Notional Amount | 308 | 329
Variable Interest Entity [Member] | Other Credit Derivatives [Member] |  |
Ambac Rating |  |
Derivative, Notional Amount | 0 | 9
Other Credit Derivatives [Member] |  |
Ambac Rating |  |
Derivative, Notional Amount | $ 257 | $ 280

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- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959 + Details
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