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	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,277,672	$ 6,324,568
Gross Unrealized Gains	262,317	74,724
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	30,342	116,634
Estimated Fair Value	6,509,647	6,282,658
Non - credit other - than - temporary Impairments	1,931	852
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,688,388	2,475,182
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,686,675
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,376
Estimated Fair Value	1,853,409
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	144,386
Estimated Fair Value	144,376
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	929,997
Estimated Fair Value	977,830
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	344,187	271,118
Gross Unrealized Gains	1	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	0
Estimated Fair Value	344,187	271,119
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,868,458	5,927,254
Gross Unrealized Gains	262,314	74,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	30,341	116,634
Estimated Fair Value	6,100,431	5,885,316
Non - credit other - than - temporary Impairments	1,931	852
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,027	126,196
Gross Unrealized Gains	2	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	65,029	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	969,764	1,405,293
Gross Unrealized Gains	19,268	857
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,950	28,427
Estimated Fair Value	981,082	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	1,879,233	1,508,377
Gross Unrealized Gains	20,139	4,886
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,538	23,894
Estimated Fair Value	1,891,834	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	148,791	131,709
Gross Unrealized Gains	532	69
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	5,198	6,901
Estimated Fair Value	144,125	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	79,402	128,415
Gross Unrealized Gains	59	9
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,877	2,176
Estimated Fair Value	77,584	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	65,027	126,196
Gross Unrealized Gains	2	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	65,029	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	30,210	32,214
Gross Unrealized Gains	22	10
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	41	70
Estimated Fair Value	30,191	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	1,686,675	1,516,877
Gross Unrealized Gains	174,110	59,853
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,376	18,105
Estimated Fair Value	1,853,409	1,558,625
Non - credit other - than - temporary Impairments	1,931	852
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	144,386	184,118
Gross Unrealized Gains	319	217
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	329	463
Estimated Fair Value	144,376	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	929,997	1,020,251
Gross Unrealized Gains	47,865	8,795
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32	36,598
Estimated Fair Value	977,830	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	6,212,645	6,198,372
Gross Unrealized Gains	262,315	74,697
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	30,342	116,634
Estimated Fair Value	6,444,618	6,156,435
Non - credit other - than - temporary Impairments	$ 1,931	$ 852