Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Total Fair Value [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Total Fair Value [Member] Futures Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Total Fair Value [Member] Credit Derivatives [Member]	Sep. 30, 2013 Successor Ambac [Member] Total Fair Value [Member] Other Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Carrying Amount [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Carrying Amount [Member] Futures Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Carrying Amount [Member] Credit Derivatives [Member]	Sep. 30, 2013 Successor Ambac [Member] Carrying Amount [Member] Other Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Assets [Member] Total Fair Value [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Assets [Member] Carrying Amount [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 1 [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 1 [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 1 [Member] Futures Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Other Contracts [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Other Assets [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 3 [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 3 [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 3 [Member] Credit Derivatives [Member]	Sep. 30, 2013 Successor Ambac [Member] Level 3 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member]	Sep. 30, 2013 Successor Ambac [Member] Municipal Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Municipal Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Municipal Obligations [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Total Fair Value [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Carrying Amount [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Level 3 [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member] Level 3 [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Foreign Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Foreign Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Foreign Obligations [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Government Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Government Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Government Obligations [Member] Level 1 [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member] Level 3 [Member]	Sep. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Commercial Asset-Backed [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Commercial Asset-Backed [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Commercial Asset-Backed [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Commercial Asset-Backed [Member] Level 3 [Member]	Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member] Level 1 [Member]	Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Other [Member] Total Fair Value [Member]	Sep. 30, 2013 Successor Ambac [Member] Other [Member] Carrying Amount [Member]	Sep. 30, 2013 Successor Ambac [Member] Other [Member] Level 2 [Member]	Sep. 30, 2013 Successor Ambac [Member] Other [Member] Level 3 [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Total Fair Value [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Total Fair Value [Member] Futures Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Total Fair Value [Member] Credit Derivatives [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Total Fair Value [Member] Other Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Carrying Amount [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Carrying Amount [Member] Futures Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Carrying Amount [Member] Credit Derivatives [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Carrying Amount [Member] Other Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Assets [Member] Total Fair Value [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Assets [Member] Carrying Amount [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Total Fair Value [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Interest Rate Swaps - Liability Position [Member] Carrying Amount [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 1 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 1 [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 1 [Member] Futures Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Other Contracts [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Other Assets [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 2 [Member] Interest Rate Swaps - Liability Position [Member] Currency Swaps [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 3 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 3 [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 3 [Member] Credit Derivatives [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Level 3 [Member] Interest Rate Swaps - Liability Position [Member] Interest Rate Swaps [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Municipal Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Municipal Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Municipal Obligations [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Total Fair Value [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Carrying Amount [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Level 3 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member] Level 3 [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Foreign Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Foreign Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Foreign Obligations [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Government Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Government Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Government Obligations [Member] Level 1 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Residential Mortgage-Backed Securities [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Residential Mortgage-Backed Securities [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Residential Mortgage-Backed Securities [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member] Level 3 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Commercial Asset-Backed [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Commercial Asset-Backed [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Commercial Asset-Backed [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Commercial Asset-Backed [Member] Level 3 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member] Level 1 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member] Level 2 [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other [Member] Total Fair Value [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other [Member] Carrying Amount [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other [Member] Level 3 [Member]
Financial assets:
Fixed income securities			$ 6,448,167																																				$ 1,444,358	$ 1,444,358	$ 1,444,358	$ 1,344,686	$ 2,423,914	$ 1,344,686	$ 2,423,914	$ 1,341,253	$ 3,433	$ 2,423,914	$ 122,499	$ 122,499	$ 122,499	$ 105,260	$ 105,260	$ 105,260	$ 35,489	$ 35,489	$ 35,216	$ 273	$ 1,555,612	$ 1,555,612	$ 1,555,612	$ 91,055	$ 91,055	$ 91,055	$ 1,059,706	$ 1,059,706	$ 992,341	$ 67,365										$ 6,329,832																																			$ 1,848,932	$ 1,848,932	$ 1,848,932	$ 1,077,972	$ 2,261,294	$ 1,077,972	$ 2,261,294	$ 1,074,316	$ 3,656	$ 2,261,294	$ 70,112	$ 70,112	$ 70,112	$ 127,283	$ 127,283	$ 127,283	$ 82,535	$ 82,535	$ 82,535	$ 1,455,582	$ 1,455,582	$ 1,455,582	$ 33,342	$ 33,342	$ 26,860	$ 6,482	$ 706,637	$ 706,637	$ 656,373	$ 50,264
Restricted cash								23,045					23,045													23,045																																																								2,290					2,290													2,290
Short term investments			549,229																																																																		549,229	549,229	538,079	11,150						661,658																																																																	661,658	661,658	657,886	3,772
Fixed income securities, pledged as collateral																																																				140,273	140,273	140,273																																																																								265,779	265,779	265,779
Other investments			237,446																																																																						237,446	237,446	237,396	50		100																																																																					100	100	100
Cash							28,551					28,551													28,551																																																								43,837					43,837													43,837
Loans at Carrying Amount					14,762,446	14,752,053						6,148	14,762,446																																																																		14,758,077	15,568,711						9,203	15,568,711
Loans					14,762,446		6,278	14,762,446																											6,278	14,762,446																																												15,359,073	7,387	15,560,051																					200,978						7,387	15,359,073
Derivative assets																	85,538	85,538													85,538																																																				1,253					1,253			124,853	124,853									1,253				124,853
Derivative assets																			(2,576)			(2,576)										(2,576)
Other assets	13,671	14,557					13,671					13,671																							13,671																																														14,557					14,557																							14,557
Total financial assets							24,026,480					24,026,350													835,208			5,913,842							17,277,430																																														24,345,454					24,355,930													1,098,328			5,544,313							17,702,813
Financial liabilities:
Obligations under investment and repurchase agreements			359,484	358,878								360,854																																																																	357,373	356,091								362,017
Obligations under investment and repurchase agreements							361,627																												361,627																																														361,905																												361,905
Liabilities subject to compromise																																																																													1,704,641	1,704,904								1,690,312
Liabilities subject to compromise																																																																																	434,823																					434,823
Long term debt, including accrued interest			954,721	940,313	15,266,893	15,023,038	1,123,202					1,225,054																							1,123,202																																										155,271	150,170	15,041,624	15,436,008	801,277					377,524																							801,277
Derivative liabilities																	(55,601)	(55,601)		1,960,732			1,960,732								(55,601)		1,960,732																																																										(73,264)	(73,264)		2,131,315			2,131,315								(73,264)		2,131,315
Derivative liabilities	202,653	213,585							2,023	202,653	155			2,023	202,653	155			251,152		86,472	251,152		86,472			2,023			155		148,131		86,472			202,653	103,021																																														213,585	220				213,585	220			390,774		90,466	390,774		90,466						220		282,022		90,466			213,585	108,752
Liabilities for net financial guarantees written							5,020,431					4,474,619																							5,020,431																																														3,091,257					7,074,808																							3,091,257
Long-term debt					15,265,390			15,266,893					15,266,893																13,642,742							1,624,151																																												15,200,538		15,414,233					15,436,008																12,457,732							2,956,501
Total financial liabilities							$ 24,219,739					$ 23,775,006													$ 2,023			$ 15,782,631							$ 8,435,085																																														$ 22,856,591					$ 27,693,765																$ 15,323,314							$ 7,533,277