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	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,556,409	$ 6,324,568
Gross Unrealized Gains	246,235	74,724
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	31,485	116,634
Estimated Fair Value	6,771,159	6,282,658
Non - credit other - than - temporary Impairments	2,476	852
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,651,683	2,475,182
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,664,096
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8,608
Estimated Fair Value	1,824,492
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,876
Estimated Fair Value	46,040
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,263,544
Estimated Fair Value	1,307,542
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	550,898	271,118
Gross Unrealized Gains	2	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	550,900	271,119
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,941,188	5,927,254
Gross Unrealized Gains	246,233	74,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	31,193	116,634
Estimated Fair Value	6,156,228	5,885,316
Non - credit other - than - temporary Impairments	2,476	852
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,323	126,196
Gross Unrealized Gains	0	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	292	0
Estimated Fair Value	64,031	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	930,171	1,405,293
Gross Unrealized Gains	18,611	857
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	9,340	28,427
Estimated Fair Value	939,442	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	1,802,775	1,508,377
Gross Unrealized Gains	12,457	4,886
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	9,549	23,894
Estimated Fair Value	1,805,683	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	126,304	131,709
Gross Unrealized Gains	1,359	69
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2,550	6,901
Estimated Fair Value	125,113	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	78,522	128,415
Gross Unrealized Gains	478	9
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	952	2,176
Estimated Fair Value	78,048	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	64,323	126,196
Gross Unrealized Gains	0	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	292	0
Estimated Fair Value	64,031	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	29,900	32,214
Gross Unrealized Gains	12	10
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	44	70
Estimated Fair Value	29,868	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	1,664,096	1,516,877
Gross Unrealized Gains	169,004	59,853
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8,608	18,105
Estimated Fair Value	1,824,492	1,558,625
Non - credit other - than - temporary Impairments	2,476	852
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,876	184,118
Gross Unrealized Gains	239	217
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	75	463
Estimated Fair Value	46,040	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	1,263,544	1,020,251
Gross Unrealized Gains	44,073	8,795
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	75	36,598
Estimated Fair Value	1,307,542	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	6,492,086	6,198,372
Gross Unrealized Gains	246,235	74,697
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	31,193	116,634
Estimated Fair Value	6,707,128	6,156,435
Non - credit other - than - temporary Impairments	$ 2,476	$ 852