Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	May 01, 2013
Assets:
Junior surplus note of Ambac Assurance Segregated Account					$ 350,000
Tolling Payment Payable to AFG	$ 30,496	$ 28,691
Successor [Member]
Assets:
Fixed income securities, at fair value	4,652,172	5,554,215
Other Investments	431,630	450,307
Total investments	5,740,791	6,500,215
Investment income due and accrued	16,532	26,023
Other assets	46,614	158,423
Total assets	23,192,374	22,635,702
Liabilities:
Current taxes	0	14,280
Total liabilities	21,547,116	20,657,678
Stockholders' equity (deficit):
Preferred stock	0	0
Common stock	453	452
Additional paid-in capital	199,560	195,267
Accumulated other comprehensive income (loss)	(52,239)	(38,990)	$ 15,215
Retained earnings	1,233,845	1,557,681
Common stock held in treasury at cost	(471)	(496)
Total Ambac Financial Group, Inc. stockholders’ equity	1,381,148	1,713,914
Total liabilities and stockholders’ equity	23,192,374	22,635,702
Ambac Financial Group, Inc Parent Company Only [Member] | Successor [Member]
Assets:
Fixed income securities, at fair value	230,055	214,023
Short-term investments, at cost (approximates fair value)	69,531	66,570
Other Investments	64,691	30,003
Total investments	364,277	310,596
Cash	3,949	32,251
Investment in subsidiaries	968,392	1,340,442
Investment income due and accrued	329	272
Current taxes receivable	29,576	28,722
Other assets	14,946	4,132
Total assets	1,381,469	1,716,415
Liabilities:
Accounts payable and other liabilities	321	2,501
Total liabilities	321	2,501
Stockholders' equity (deficit):
Preferred stock	0	0
Common stock	453	452
Additional paid-in capital	199,560	195,267
Accumulated other comprehensive income (loss)	(52,239)	(38,990)
Retained earnings	1,233,845	1,557,681
Common stock held in treasury at cost	(471)	(496)
Total Ambac Financial Group, Inc. stockholders’ equity	1,381,148	1,713,914	$ 1,684,799	$ 1,399,105
Total liabilities and stockholders’ equity	$ 1,381,469	$ 1,716,415