Investments - Summary of Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]
Investment [Line Items]
Fixed income securities	$ 53,301	$ 83,732	$ 85,364	$ 118,097	$ 292,483
Short-term investments	626	788	431	677	1,188
Loans	109	193	137	146	558
Investment expense	(2,260)	(3,726)	(1,854)	(2,549)	(4,198)
Securities available-for-sale and short-term	51,776	80,987	84,078	116,371	290,031
Other investments	350	(2,665)		369
Total net investment income	$ 52,126	$ 78,322	$ 84,078	$ 116,740	$ 290,031