Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail2) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investment [Line Items]
Securities fair value	$ 17	$ 8
Market Value of Secured Note Collateral	0	178
Fair Value of Securities Held by Ambac UK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note	669
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	120	140
Standard & Poor's, B Rating
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	757
Standard & Poor's, B Rating | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	754
Standard & Poor's, B Rating | Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	318
Standard & Poor's, B Rating | Municipal Bonds [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	316
Standard & Poor's, B Rating | Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0
Standard & Poor's, B Rating | Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	439
Standard & Poor's, B Rating | Mortgage and Asset-Backed Securities [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	439
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2	6
Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2	6
Standard & Poor's, BBB- Rating [Member] | Corporate Debt Securities [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0	0
Standard & Poor's, C Rating | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1
Standard & Poor's, C Rating | Municipal Bonds [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1
Standard & Poor's, C Rating | Corporate Debt Securities [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		0
Standard & Poor's, CCC+ Rating
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,273
Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,266
Standard & Poor's, CCC+ Rating | Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		327
Standard & Poor's, CCC+ Rating | Municipal Bonds [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		320
Standard & Poor's, CCC+ Rating | Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		465
Standard & Poor's, CCC+ Rating | Corporate Debt Securities [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		465
Standard & Poor's, CCC+ Rating | Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		481
Standard & Poor's, CCC+ Rating | Mortgage and Asset-Backed Securities [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 481
Standard & Poor's, A- Rating | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Standard & Poor's, A- Rating | Municipal Bonds [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Standard & Poor's, A- Rating | Corporate Debt Securities [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 0