Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) (Variable interest entity [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 93,517,995	$ 111,748,270
Insurance Assets	1,840,265	2,196,161
Insurance Liabilities	7,463,635	9,207,107
Derivative Liabilities	201,434	183,666
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	56,421,767	73,854,544
Insurance Assets	1,298,086	1,627,129
Insurance Liabilities	6,830,277	8,506,417
Derivative Liabilities	172,771	168,816
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	37,096,228	37,893,726
Insurance Assets	542,179	569,032
Insurance Liabilities	633,358	700,690
Derivative Liabilities	28,663	14,850
Collateralized debt obligations [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	10,176,522	14,093,243
Insurance Assets	9,673	15,096
Insurance Liabilities	13,328	105,219
Derivative Liabilities	113,057	120,614
Residential mortgage-backed securities [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	24,008,616	30,307,753
Insurance Assets	603,867	783,329
Insurance Liabilities	3,969,336	5,396,711
Derivative Liabilities
Mortgage-backed-commercial [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	643,387	685,870
Insurance Assets
Insurance Liabilities
Derivative Liabilities	2,418	6,241
Other consumer asset-backed [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	5,895,377	7,851,980
Insurance Assets	101,494	129,234
Insurance Liabilities	1,042,522	1,315,146
Derivative Liabilities	45,610	35,583
Other commercial asset-backed [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	10,192,858	13,363,434
Insurance Assets	451,048	559,884
Insurance Liabilities	1,215,074	1,045,477
Derivative Liabilities	7,293	4,541
Other [Member] \| Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	5,505,007	7,552,264
Insurance Assets	132,004	139,586
Insurance Liabilities	590,017	643,864
Derivative Liabilities	$ 4,393	$ 1,837

Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) (Variable interest entity [Member], USD $)
In Thousands, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2011

Variable Interest Entities [Line Items]

Maximum Exposure To Loss

$ 93,517,995

$ 111,748,270

Insurance Assets

1,840,265

2,196,161

Insurance Liabilities

7,463,635

9,207,107