Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 Successor Ambac [Member]	Jun. 30, 2013 Successor Ambac [Member] Municipal Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Foreign Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] U.S. government obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] U.S. Agency Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member]	Jun. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Asset-Backed Securities [Member]	Jun. 30, 2013 Successor Ambac [Member] Collateralized Investments [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Municipal Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Foreign Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. government obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] U.S. Agency Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Residential Mortgage-Backed Securities [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Asset-Backed Securities [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,676,216	$ 1,696,216	$ 1,008,545	$ 109,147	$ 251,513	$ 53,390	$ 1,500,052	$ 47,645	$ 1,009,708		$ 4,751,824	$ 1,662,124	$ 999,554	$ 67,347	$ 127,037	$ 79,295	$ 1,096,202	$ 32,855	$ 687,410
Amortized cost, short-term investments	611,174										661,219
Investment other than available for sale securities pledged as collateral amortized cost basis	6,287,390										5,413,043
Amortized cost basis of securities pledged as collateral	164,340				164,340						265,517				265,517
Amortized Cost available-for-sale on Investment	6,451,730						1,500,052	47,645			5,678,560
Gross Unrealized Gains	46,758	77	479	136	70	4	44,050	151	1,791		728,286	187,191	87,535	2,765	872	3,240	379,935	1,015	65,733
Short-term	7										439
Investment other than available for sale securities pledged as collateral gross unrealized gain	46,765										728,725
Gross unrealized gains on securities pledged as collateral	1				1						262				262
Gross Unrealized Gains On available-for-sale Investment	46,766										728,987
Gross Unrealized Losses	137,699	59,103	26,437	4,092	745	148	23,426	178	23,570		77,715	383	9,117		626		20,555	528	46,506
Other short-term investments gross unrealized losses	4
Investment other than available for sale securities pledged as collateral gross unrealized losses	137,703										77,715
Gross unrealized losses on securities pledged as collateral	46				46
Gross Unrealized Loss On available-for-sale Investment	137,749										77,715
Estimated Fair Value	5,585,275	1,637,190	982,587	105,191	250,838	53,246	1,520,676	47,618	987,929		5,402,395	1,848,932	1,077,972	70,112	127,283	82,535	1,455,582	33,342	706,637
Short-term Investments	611,177										661,658
Non - credit other - than - temporary Impairments	8						8				6,892						6,892
Investment other than available for sale securities pledged as collateral	6,196,452										6,064,053
Non-credit other-than-temporary Impairments
Non - credit other - than - temporary Impairments	8						8				6,892						6,892
Fair value of securities pledged as collateral	164,295				164,295						265,779				265,779
Non - credit other - than - temporary Impairments	8						8				6,892						6,892
Total available-for-sale investments	6,360,747						1,520,676	47,618			6,329,832
Non - credit other - than - temporary Impairments	$ 8						$ 8				$ 6,892						$ 6,892