Investments - Summary of Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Apr. 30, 2013	Sep. 30, 2013	Sep. 30, 2014
Successor Ambac [Member]
Investment [Line Items]
Fixed income securities	$ 84,339	$ 53,301		$ 83,732	$ 233,878
Short-term investments	832	626		788	2,000
Loans	123	109		193	415
Investment expense	(2,413)	(2,260)		(3,726)	(7,723)
Securities available-for-sale and short-term	82,881	51,776		80,987	228,570
Other investments	700	350		(2,665)	5,905
Total net investment income	83,581	52,126		78,322	234,475
Predecessor Ambac [Member]
Investment [Line Items]
Fixed income securities			118,097
Short-term investments			677
Loans			146
Investment expense			(2,549)
Securities available-for-sale and short-term			116,371
Other investments			369
Total net investment income			116,740
Gains (losses) on securities held as of reporting date [Member] | Successor Ambac [Member]
Investment [Line Items]
Other investments	$ 329	$ 1,577	$ (78)	$ (784)	$ 5,965