Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Stockholders' Equity (Detail) - USD ($) $ in Millions	12 Months Ended	18 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	$ (125)	$ 274	$ (335)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	(12)	(12)	(4)
Issuance of common stock	(3)	(1)	(2)
Payments for Repurchase of Warrants				$ 8
AMPS Impact on Stockholders Equity	19	(297)
Ending balance	1,536	1,633	1,645	1,978
Stockholders' Equity Attributable to Parent	1,477	1,592
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(216)	267	(329)
Stock-based compensation	0	0
Issuance of common stock	(3)	(1)	(2)
AMPS Impact on Stockholders Equity	0	(82)
Ending balance	1,203	1,421	1,234	1,558
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	91	6	(7)
Stock-based compensation	0	0
Issuance of common stock	0	0
AMPS Impact on Stockholders Equity	0	0
Ending balance	42	(49)	(52)	(39)
Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	0	0
Issuance of common stock	0	0
AMPS Impact on Stockholders Equity	0	0
Ending balance	0	0	0	0
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	0	0
Issuance of common stock	0	0
AMPS Impact on Stockholders Equity	0	0
Ending balance	0	0	0	0
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	(12)	(12)	(4)
Issuance of common stock	0	0
AMPS Impact on Stockholders Equity	0	8
Ending balance	232	219	200	195
Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	0	0
Issuance of common stock	0	0	0
AMPS Impact on Stockholders Equity	0	0
Ending balance	0	0	0	0
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	0	0
Issuance of common stock	0	0	0
AMPS Impact on Stockholders Equity	19	(223)
Ending balance	60	41	264	264
Ambac Financial Group, Inc Parent Company Only [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(125)	192	(335)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	(12)	(12)	(4)
Issuance of common stock	(3)	(1)	(2)
AMPS Impact on Stockholders Equity		8
Stockholders' Equity Attributable to Parent	1,477	1,592	1,381	1,714
Ambac Financial Group, Inc Parent Company Only [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(216)	186	(329)
Issuance of common stock	(3)	(1)	(2)
Stockholders' Equity Attributable to Parent	1,203	1,421	1,234	1,558
Ambac Financial Group, Inc Parent Company Only [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	91	6	(7)
Stockholders' Equity Attributable to Parent	42	(49)	(52)	(39)
Ambac Financial Group, Inc Parent Company Only [Member] \| Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0	0	0
Ambac Financial Group, Inc Parent Company Only [Member] \| Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0	0	0
Ambac Financial Group, Inc Parent Company Only [Member] \| Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	(12)	(12)	(4)
AMPS Impact on Stockholders Equity		8
Stockholders' Equity Attributable to Parent	232	219	200	195
Ambac Financial Group, Inc Parent Company Only [Member] \| Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation		0
Issuance of common stock	0	0	0
Stockholders' Equity Attributable to Parent	$ 0	0	0	$ 0
Accounting Standards Update 2016-01 [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		3
Accounting Standards Update 2016-01 [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		(3)
Accounting Standards Update 2016-01 [Member] \| Parent [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		3
Accounting Standards Update 2016-01 [Member] \| Parent [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ (3)
Accounting Standards Update 2018-02 [Domain] \| Ambac Financial Group, Inc Parent Company Only [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			0
Accounting Standards Update 2018-02 [Domain] \| Ambac Financial Group, Inc Parent Company Only [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			7
Accounting Standards Update 2018-02 [Domain] \| Ambac Financial Group, Inc Parent Company Only [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ (7)

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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