Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial liabilities:
Derivative liabilities	$ 15,349	$ 34,543
Successor [Member]
Financial assets:
Fixed income securities	6,049,908	5,334,120
Short term investments	430,788	225,789
Other investments	450,307	310,600
Other assets	7,382	8,696
Financial liabilities:
Borrowings under Guaranteed Investment Agreements	82,358	100,358
Long term debt, including accrued interest	1,114,405	1,124,950
Successor [Member] | Variable Interest Entity [Member]
Financial assets:
Loans at Carrying Amount	10,658,963	11,690,324
Financial liabilities:
Long term debt, including accrued interest	11,155,936	12,327,960
Successor [Member] | Carrying Amount [Member]
Financial assets:
Cash	91,025	35,744
Loans at Carrying Amount		5,206
Loans	4,160
Other assets	7,382	8,696
Total financial assets	20,047,333	20,064,063
Financial liabilities:
Borrowings under Guaranteed Investment Agreements	82,358	100,358
Long term debt, including accrued interest	1,536,352	1,481,045
Liabilities for net financial guarantees written	3,009,943	2,033,484
Total financial liabilities	18,182,476	18,224,608
Successor [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	4,873	5,822
Loans at Carrying Amount	10,658,963	11,690,324
Financial liabilities:
Long-term debt	11,155,936	12,327,960
Successor [Member] | Carrying Amount [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets	536	109
Successor [Member] | Carrying Amount [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	15,349	34,543
Successor [Member] | Total Fair Value [Member]
Financial assets:
Cash	91,025	35,744
Loans	4,066	5,128
Other assets	7,382	8,696
Total financial assets	20,032,169	20,051,480
Financial liabilities:
Obligations under investment and repurchase agreements	82,333	101,400
Long term debt, including accrued interest	1,494,340	1,235,721
Liabilities for net financial guarantees written	4,490,070	2,325,859
Total financial liabilities	19,620,566	18,272,701
Successor [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	4,873	5,822
Loans	10,658,963	11,690,324
Financial liabilities:
Long-term debt	11,155,936	12,327,960
Successor [Member] | Total Fair Value [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets	536	109
Successor [Member] | Total Fair Value [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	15,349	34,543
Successor [Member] | Investments Measured at NAV as a Practical Expedient [Member]
Financial assets:
Other investments	336,513	239,516
Successor [Member] | Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	77,206	84,886
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	(61,839)	(52,128)
Successor [Member] | Other Assets [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	77,206	84,886
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	(61,839)	(52,128)
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	365,776	370,943
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,078,601	1,965,265
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	80,407
Financial liabilities:
Derivative liabilities		(36,862)
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	365,776	370,943
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,078,601	1,965,265
Successor [Member] | Interest Rate Swaps - Liability Position [Member] | Total Fair Value [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	80,407
Financial liabilities:
Derivative liabilities		(36,862)
Successor [Member] | Level 1 [Member]
Financial assets:
Cash	46,587	35,744
Loans	0	0
Other assets	0	0
Total financial assets	650,457	463,868
Financial liabilities:
Obligations under investment and repurchase agreements	0	0
Long term debt, including accrued interest	0	0
Liabilities for net financial guarantees written	0	0
Total financial liabilities	0	0
Successor [Member] | Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	4,873	5,822
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Successor [Member] | Level 1 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets	536	109
Successor [Member] | Level 1 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Successor [Member] | Level 1 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	0	0
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0	0
Successor [Member] | Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	0	0
Successor [Member] | Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Successor [Member] | Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities		0
Successor [Member] | Level 2 [Member]
Financial assets:
Cash	44,438	0
Loans	0	0
Other assets	0	0
Total financial assets	4,914,330	4,490,636
Financial liabilities:
Obligations under investment and repurchase agreements	0	0
Long term debt, including accrued interest	1,147,728	132,837
Liabilities for net financial guarantees written	0	0
Total financial liabilities	11,958,793	11,400,158
Successor [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term debt	8,573,716	9,147,790
Successor [Member] | Level 2 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets	0	0
Successor [Member] | Level 2 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Successor [Member] | Level 2 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	16,950	21,848
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	(61,839)	(52,128)
Successor [Member] | Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	220,587	243,256
Successor [Member] | Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	2,078,601	1,965,265
Successor [Member] | Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	80,407
Financial liabilities:
Derivative liabilities		(36,862)
Successor [Member] | Level 3 [Member]
Financial assets:
Cash	0	0
Loans	4,066	5,128
Other assets	7,382	8,696
Total financial assets	14,130,870	14,857,460
Financial liabilities:
Obligations under investment and repurchase agreements	82,333	101,400
Long term debt, including accrued interest	346,612	1,102,884
Liabilities for net financial guarantees written	4,490,070	2,325,859
Total financial liabilities	7,661,773	6,872,543
Successor [Member] | Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	10,658,963	11,690,324
Financial liabilities:
Long-term debt	2,582,220	3,180,170
Successor [Member] | Level 3 [Member] | Futures Contracts [Member]
Financial assets:
Derivative assets	0	0
Successor [Member] | Level 3 [Member] | Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	15,349	34,543
Successor [Member] | Level 3 [Member] | Other Assets [Member] | Interest Rate Swaps [Member]
Financial assets:
Derivative assets	60,256	63,038
Derivative assets - interest rate swaps - liability position		0
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0	0
Successor [Member] | Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	145,189	127,687
Successor [Member] | Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Interest Rate Swaps [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Successor [Member] | Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities		0
Successor [Member] | Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	374,368	420,770
Successor [Member] | Municipal Bonds [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	374,368	420,770
Successor [Member] | Municipal Bonds [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Municipal Bonds [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	374,368	420,770
Successor [Member] | Municipal Bonds [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Corporate Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	1,802,165	1,593,669
Successor [Member] | Corporate Obligations [Member] | Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,622,566	2,588,556
Successor [Member] | Corporate Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	1,802,165	1,593,669
Successor [Member] | Corporate Obligations [Member] | Total Fair Value [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,622,566	2,588,556
Successor [Member] | Corporate Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Corporate Obligations [Member] | Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Corporate Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	1,802,165	1,593,669
Successor [Member] | Corporate Obligations [Member] | Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Corporate Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Corporate Obligations [Member] | Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Fixed income securities	2,622,566	2,588,556
Successor [Member] | Foreign Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	43,135	96,306
Successor [Member] | Foreign Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	43,135	96,306
Successor [Member] | Foreign Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	42,212	87,808
Successor [Member] | Foreign Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	923	8,498
Successor [Member] | Foreign Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	36,186	26,687
Fixed income securities, pledged as collateral	64,905	64,555
Successor [Member] | US Government Debt Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	36,186	26,687
Fixed income securities, pledged as collateral	64,905	64,555
Successor [Member] | US Government Debt Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	36,186	26,687
Fixed income securities, pledged as collateral	64,905	64,555
Successor [Member] | US Government Debt Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	0	0
Fixed income securities, pledged as collateral	0	0
Successor [Member] | US Government Debt Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Fixed income securities, pledged as collateral	0	0
Successor [Member] | U.S. Agency Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	4,060	4,212
Successor [Member] | U.S. Agency Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	4,060	4,212
Successor [Member] | U.S. Agency Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | U.S. Agency Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	4,060	4,212
Successor [Member] | U.S. Agency Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	2,351,595	1,977,338
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	2,351,595	1,977,338
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	1,654,882	1,488,454
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	696,713	488,884
Successor [Member] | Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	113,923	84,267
Successor [Member] | Collateralized Debt Obligations [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	113,923	84,267
Successor [Member] | Collateralized Debt Obligations [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Collateralized Debt Obligations [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	113,923	84,267
Successor [Member] | Collateralized Debt Obligations [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Fixed income securities	828,783	840,527
Successor [Member] | Asset-backed Securities [Member] | Total Fair Value [Member]
Financial assets:
Fixed income securities	828,783	840,527
Successor [Member] | Asset-backed Securities [Member] | Level 1 [Member]
Financial assets:
Fixed income securities	0	0
Successor [Member] | Asset-backed Securities [Member] | Level 2 [Member]
Financial assets:
Fixed income securities	762,793	840,527
Successor [Member] | Asset-backed Securities [Member] | Level 3 [Member]
Financial assets:
Fixed income securities	65,990	0
Successor [Member] | Short-Term [Member] | Carrying Amount [Member]
Financial assets:
Short term investments	430,788	225,789
Successor [Member] | Short-Term [Member] | Total Fair Value [Member]
Financial assets:
Short term investments	430,788	225,789
Successor [Member] | Short-Term [Member] | Level 1 [Member]
Financial assets:
Short term investments	371,367	197,398
Successor [Member] | Short-Term [Member] | Level 2 [Member]
Financial assets:
Short term investments	59,421	28,391
Successor [Member] | Short-Term [Member] | Level 3 [Member]
Financial assets:
Short term investments	0	0
Successor [Member] | Other [Member] | Carrying Amount [Member]
Financial assets:
Other investments	450,307	310,600
Successor [Member] | Other [Member] | Total Fair Value [Member]
Financial assets:
Other investments	435,237	298,095
Successor [Member] | Other [Member] | Level 1 [Member]
Financial assets:
Other investments	83,791	45,745
Successor [Member] | Other [Member] | Level 2 [Member]
Financial assets:
Other investments	0	0
Successor [Member] | Other [Member] | Level 3 [Member]
Financial assets:
Other investments	$ 14,934	$ 12,834