Investments - Summary of Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	2 Months Ended	3 Months Ended	6 Months Ended	1 Months Ended	4 Months Ended	2 Months Ended	3 Months Ended	6 Months Ended	1 Months Ended	4 Months Ended
	Jun. 30, 2013 Successor Ambac [Member]	Jun. 30, 2014 Successor Ambac [Member]	Jun. 30, 2014 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Jun. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member]	Jun. 30, 2014 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member]	Jun. 30, 2014 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member]	Apr. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Predecessor Ambac [Member]
Investment [Line Items]
Fixed income securities	$ 30,431	$ 78,554	$ 149,539	$ 31,766	$ 118,097
Short-term investments	162	865	1,168	82	677
Loans	84	175	292	38	146
Investment expense	(1,466)	(2,712)	(5,310)	(572)	(2,549)
Securities available-for-sale and short-term	29,211	76,882	145,689	31,314	116,371
Other investments	(3,015)	3,211	5,205	912	369	(3,015)	3,211	5,636	912	369
Total net investment income	$ 26,196	$ 80,093	$ 150,894	$ 32,226	$ 116,740