Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (Successor Ambac [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,462,103	$ 6,324,568
Gross Unrealized Gains	130,258	74,724
Gross Unrealized Losses	83,446	116,634
Estimated Fair Value	6,508,915	6,282,658
Non - credit other - than - temporary Impairments	834	852
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,546,762	2,475,182
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,578,205
Estimated Fair Value	1,655,658
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,780
Estimated Fair Value	177,339
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,237,667
Estimated Fair Value	1,221,993
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	215,103	271,118
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	0
Estimated Fair Value	215,103	271,119
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,121,910	5,927,254
Gross Unrealized Gains	130,225	74,696
Gross Unrealized Losses	83,446	116,634
Estimated Fair Value	6,168,689	5,885,316
Non - credit other - than - temporary Impairments	834	852
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125,090	126,196
Gross Unrealized Gains	33	27
Gross Unrealized Losses	0	0
Estimated Fair Value	125,123	126,223
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,189,334	1,405,293
Gross Unrealized Gains	12,932	857
Gross Unrealized Losses	16,978	28,427
Estimated Fair Value	1,185,288	1,377,723
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,652,302	1,508,377
Gross Unrealized Gains	9,535	4,886
Gross Unrealized Losses	14,253	23,894
Estimated Fair Value	1,647,584	1,489,369
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	146,189	131,709
Gross Unrealized Gains	673	69
Gross Unrealized Losses	4,818	6,901
Estimated Fair Value	142,044	124,877
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	108,546	128,415
Gross Unrealized Gains	75	9
Gross Unrealized Losses	1,663	2,176
Estimated Fair Value	106,958	126,248
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | U.S. Government Obligations [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125,090	126,196
Gross Unrealized Gains	33	27
Gross Unrealized Losses	0	0
Estimated Fair Value	125,123	126,223
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,887	32,214
Gross Unrealized Gains	3	10
Gross Unrealized Losses	65	70
Estimated Fair Value	31,825	32,154
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,578,205	1,516,877
Gross Unrealized Gains	97,861	59,853
Gross Unrealized Losses	20,408	18,105
Estimated Fair Value	1,655,658	1,558,625
Non - credit other - than - temporary Impairments	834	852
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,780	184,118
Gross Unrealized Gains	202	217
Gross Unrealized Losses	643	463
Estimated Fair Value	177,339	183,872
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,237,667	1,020,251
Gross Unrealized Gains	8,944	8,795
Gross Unrealized Losses	24,618	36,598
Estimated Fair Value	1,221,993	992,448
Non - credit other - than - temporary Impairments	0	0
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,337,013	6,198,372
Gross Unrealized Gains	130,225	74,697
Gross Unrealized Losses	83,446	116,634
Estimated Fair Value	6,383,792	6,156,435
Non - credit other - than - temporary Impairments	$ 834	$ 852