Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Stockholders' Equity (Detail) - Successor [Member] - USD ($)	12 Months Ended			30 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	$ (335,376,000)	$ 20,112,000	$ 287,252,000
Stock-based compensation	(4,293,000)	(5,253,000)	(3,105,000)
Cost of shares (acquired) issued under equity plan	(1,547,000)	(505,000)	(374,000)
Payments for Repurchase of Warrants	0	2,717,000	5,375,000	$ 8,092,000
Stock Issued During Period, Value, New Issues	1,000	2,000
Noncontrolling Interest, Decrease from Deconsolidation		(2,469,000)
Proceeds from Warrant Exercises	0	2,000	3,000
Warrants exercised		2,000	3,000
Ending balance	1,645,258,000	1,978,024,000	1,958,346,000	1,645,258,000
Stockholders' Equity Attributable to Parent	1,381,148,000	1,713,914,000		1,381,148,000
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(328,715,000)	74,843,000	493,403,000
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	(1,572,000)	(127,000)	(312,000)
Payments for Repurchase of Warrants	0	1,916,000	3,942,000
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		0	0
Ending balance	1,233,845,000	1,557,681,000	1,478,439,000	1,233,845,000
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(6,661,000)	(54,205,000)	(205,068,000)
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants	0	0	0
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		0	0
Ending balance	(52,239,000)	(38,990,000)	15,215,000	(52,239,000)
Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants	0	0	0
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		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
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants	0	0	0
Stock Issued During Period, Value, New Issues	1,000	2,000
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		0	0
Ending balance	453,000	452,000	450,000	453,000
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
Stock-based compensation	(4,293,000)	(5,253,000)	(3,105,000)
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants	0	801,000	1,433,000
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		2,000	3,000
Ending balance	199,560,000	195,267,000	190,813,000	199,560,000
Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	0	0
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	25,000	(378,000)	(62,000)
Payments for Repurchase of Warrants	0	0	0
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		0
Warrants exercised		0	0
Ending balance	(471,000)	(496,000)	(118,000)	(471,000)
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0	(526,000)	(1,083,000)
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants	0	0	0
Stock Issued During Period, Value, New Issues	0	0
Noncontrolling Interest, Decrease from Deconsolidation		(2,469,000)
Warrants exercised		0	0
Ending balance	264,110,000	264,110,000	273,547,000	264,110,000
Ambac Financial Group, Inc Parent Company Only [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(335,376,000)	20,638,000	288,335,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(137,000)	6,442,000
Stock-based compensation	(4,293,000)	(5,253,000)	(3,105,000)
Cost of shares (acquired) issued under equity plan	(1,547,000)	(505,000)	(374,000)
Payments for Repurchase of Warrants	0	(2,717,000)	(5,375,000)
Stock Issued During Period, Value, New Issues	1,000	2,000
Proceeds from Warrant Exercises	0	2,000	3,000
Warrants exercised		2,000	3,000
Stockholders' Equity Attributable to Parent	1,381,148,000	1,713,914,000	1,684,799,000	1,381,148,000	$ 1,399,105,000
Ambac Financial Group, Inc Parent Company Only [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(328,715,000)	74,843,000	493,403,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(137,000)	6,442,000
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	(1,572,000)	(127,000)	(312,000)
Payments for Repurchase of Warrants		(1,916,000)	(3,942,000)
Stock Issued During Period, Value, New Issues		0
Warrants exercised		0	0
Stockholders' Equity Attributable to Parent	1,233,845,000	1,557,681,000	1,478,439,000	1,233,845,000	989,290,000
Ambac Financial Group, Inc Parent Company Only [Member] | AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	(6,661,000)	(54,205,000)	(205,068,000)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Stock-based compensation	0	0	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants		0	0
Stock Issued During Period, Value, New Issues		0
Warrants exercised		0	0
Stockholders' Equity Attributable to Parent	(52,239,000)	(38,990,000)	15,215,000	(52,239,000)	220,283,000
Ambac Financial Group, Inc Parent Company Only [Member] | 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	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants		0	0
Stock Issued During Period, Value, New Issues		0
Warrants exercised		0	0
Stockholders' Equity Attributable to Parent	0	0	0	0	0
Ambac Financial Group, Inc Parent Company Only [Member] | 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	0
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants		0	0
Stock Issued During Period, Value, New Issues	1,000	2,000
Warrants exercised		0	0
Stockholders' Equity Attributable to Parent	453,000	452,000	450,000	453,000	450,000
Ambac Financial Group, Inc Parent Company Only [Member] | 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	(4,293,000)	(5,253,000)	(3,105,000)
Cost of shares (acquired) issued under equity plan	0	0	0
Payments for Repurchase of Warrants		(801,000)	(1,433,000)
Stock Issued During Period, Value, New Issues		0
Warrants exercised		2,000	3,000
Stockholders' Equity Attributable to Parent	199,560,000	195,267,000	190,813,000	199,560,000	189,138,000
Ambac Financial Group, Inc Parent Company Only [Member] | 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	0
Cost of shares (acquired) issued under equity plan	25,000	(378,000)	(62,000)
Payments for Repurchase of Warrants		0	0
Stock Issued During Period, Value, New Issues		0
Warrants exercised		0	0
Stockholders' Equity Attributable to Parent	(471,000)	$ (496,000)	$ (118,000)	$ (471,000)	$ (56,000)
Accounting Standards Update 2018-02 [Domain]
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] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	6,588,000
Accounting Standards Update 2018-02 [Domain] | AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(6,588,000)
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	6,588,000
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	$ (6,588,000)