Derivative Instruments - Summary of Net Par Outstanding for CDS Contracts (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Ambac Rating
Notional outstanding	$ 11,281,777	$ 14,166,612
AAA Rating [Member]
Ambac Rating
Notional outstanding	678,483	1,211,598
AA Rating [Member]
Ambac Rating
Notional outstanding	5,955,118	7,442,106
A Rating [Member]
Ambac Rating
Notional outstanding	3,684,193	4,704,759
BBB Rating [Member]
Ambac Rating
Notional outstanding	672,293	518,142
Below investment grade rating [Member]
Ambac Rating
Notional outstanding	291,690	290,007
CLO [Member]
Ambac Rating
Notional outstanding	6,155,767	8,228,577
CLO [Member] | AAA Rating [Member]
Ambac Rating
Notional outstanding	166,200	297,741
CLO [Member] | AA Rating [Member]
Ambac Rating
Notional outstanding	4,676,362	6,193,522
CLO [Member] | A Rating [Member]
Ambac Rating
Notional outstanding	1,313,205	1,737,314
Other [Member]
Ambac Rating
Notional outstanding	5,126,010	5,938,035
Other [Member] | AAA Rating [Member]
Ambac Rating
Notional outstanding	512,283	913,857
Other [Member] | AA Rating [Member]
Ambac Rating
Notional outstanding	1,278,756	1,248,584
Other [Member] | A Rating [Member]
Ambac Rating
Notional outstanding	2,370,988	2,967,445
Other [Member] | BBB Rating [Member]
Ambac Rating
Notional outstanding	672,293	518,142
Other [Member] | Below investment grade rating [Member]
Ambac Rating
Notional outstanding	$ 291,690	$ 290,007