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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 1,164 |
Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | $ 2,105
Municipal Bonds [Member] | Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 187 |
Municipal Bonds [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 849
Corporate Debt Securities [Member] | Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 535 |
Corporate Debt Securities [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 656
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 442 |
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 599
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 11 | 16
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 11 | 16
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 0 | 0
Ambac Assurance Corporation [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 2,089
Ambac Assurance Corporation [Member] | Standard & Poor's, CCC Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 833
Ambac Assurance Corporation [Member] | Standard & Poor's, CCC Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 656
Ambac Assurance Corporation [Member] | Standard & Poor's, CCC Rating [Member] | Mortgage and Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | $ 599
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 1,153 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 176 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 535 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Mortgage and Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 442 |

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- Definition Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-12C.Column C.7) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14.Column E.2,3) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12.Column C.3,4) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 5 -Subparagraph (SX 210.12-13.Column C.6) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611282-123010 + Details
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