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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

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