Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 01, 2013
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued	45,571,743	45,365,170
Treasury stock, shares	16,343	28,892
Common Stock, Shares, Outstanding	45,555,400	45,336,278
Assets:
Debt Securities, Available-for-sale	$ 3,314
Junior surplus note of Ambac Assurance Segregated Account					$ 350
Short-term investments, at cost (approximates fair value)	3,187	$ 3,451
Other Investments	478	391
Total investments	3,792	3,937
Other assets	95	138
Total assets	13,320	14,589
Tolling Payment Payable to AFG	28	44
Liabilities:
Total liabilities	11,783	12,956
Stockholders' equity (deficit):
Preferred stock	0	0
Common stock	0	0
Additional paid-in capital	232	219
Accumulated other comprehensive income (loss)	42	(49)	$ (52)
Retained earnings	1,203	1,421
Common stock held in treasury at cost	0	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,477	1,592
Total liabilities and stockholders’ equity	13,320	14,589
Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	71	151
Other Investments	46	40
Total investments	434	381
Cash	9	15
Investment in subsidiaries	993	1,148
Investment income due and accrued	1	1
Current taxes receivable	30	44
Other assets	11	3
Total assets	1,478	1,593
Liabilities:
Accounts payable and other liabilities	2	1
Total liabilities	2	1
Stockholders' equity (deficit):
Preferred stock	0	0
Common stock	0	0
Additional paid-in capital	232	219
Accumulated other comprehensive income (loss)	42	(49)
Retained earnings	1,203	1,421
Common stock held in treasury at cost	0	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,477	1,592	$ 1,381	$ 1,714
Total liabilities and stockholders’ equity	1,478	1,593
Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale	653	430
Short-term investments, at cost (approximates fair value)	653	430
Short-term Investments [Member] | Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	318	193
Fixed Income Securities [Member]
Assets:
Debt Securities, Available-for-sale	2,577	3,116
Short-term investments, at cost (approximates fair value)	2,450	3,021
Fixed Income Securities [Member] | Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Debt Securities, Available-for-sale	$ 70	$ 148