Background and Business Description - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended	8 Months Ended
	Sep. 30, 2014	Dec. 31, 2014 Rate Segment	Dec. 31, 2013	Apr. 30, 2013	Dec. 31, 2010	Dec. 31, 2013	May 01, 2013
Background And Basis Of Presentation [Line Items]
Unpaid as a Percentage of Permitted Policy Claims		55.00%
Entity Incorporation, Date of Incorporation		Apr. 29, 1991
Number of reportable segments		2
Par Value of Owner Trust Certificate		$ 74,794,000
Par Value of Private Placement		299,175,000
Private Placement Par Value Proceeds		80.00%
Private Placement Maturity Date		Aug. 28, 2039
Interest on junior surplus notes		5.10%
Investment Warrants Expiration Date		Apr. 30, 2023
Transaction Limit		5,000,000
Accrued rate of interest on outstanding policy obligations (percent)		5.10%
Payment of permitted policy claim (percent)	25.00%	25.00%
Equalizing payment on unpaid permitted policy claim (percent)		26.67%
December 2014 Payment of Deferred Amounts		1,137,471,000
Capital Stock Authorized				150,000,000
Common stock, shares authorized		130,000,000	130,000,000	130,000,000		130,000,000
Common Stock, Par or Stated Value Per Share		$ 0.01	$ 0.01	$ 0.01		$ 0.01
Preferred stock, shares authorized		20,000,000	20,000,000	20,000,000		20,000,000
Preferred Stock, Par or Stated Value Per Share		$ 0.01	$ 0.01	$ 0.01		$ 0.01
Common stock, shares issued		45,005,932	45,003,461	45,000,000		45,003,461
Warrants Distributed In Bankruptcy				5,047,138
Warrants issued exercise price				$ 16.67
Mediation Agreement Cash Grant				30,000,000
Junior surplus note of Ambac Assurance Segregated Account							350,000,000
Common Stock Voting Restrictions		10.00%	10.00%
Common Stock Voting Restriction Less One Vote		10.00%
Common Stock Transfer Restrictions		5.00%
Common Stock Additional Transfer Restrictions		5.00%
November 2014 Payment of Surplus Notes		413,587,000
Ambac Assurance [Member]
Background And Basis Of Presentation [Line Items]
Net par exposure for policies allocated to the Segregated Account		18,808,470,000
Issuance of secured note to segregated account					2,000,000,000
Secured note maturity year					2050
Interest on the secured note accrues					4.50%
Available secured notes including capitalized interest		0
Minimum surplus amount		100,000,000
Maximum Reimbursement Of Operating Expenses				5,000,000
Successor [Member]
Background And Basis Of Presentation [Line Items]
Proceeds from the sale of Junior Surplus Notes of the Segregated Account		224,262,000				0
Common stock, shares issued				45,000,000
Warrants Distributed In Bankruptcy				5,047,138
Two Thousand And Seventeen [Member] | Ambac Assurance [Member]
Background And Basis Of Presentation [Line Items]
Maximum Reimbursement Of Operating Expenses after 2017				$ 4,000,000