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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - Successor [Member] - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 5,298,840 | $ 4,939,325
Gross Unrealized Gains | 158,969 | 242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,181 | 32,269
Estimated Fair Value | 5,386,628 | 5,150,018
Non - credit other - than - temporary Impairments | 27,439 | 7,773
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,679,895 | 2,743,050
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 364,784 | 360,069
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 4
Estimated Fair Value | 364,780 | 360,065
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 4,869,500 | 4,514,878
Gross Unrealized Gains | 158,553 | 242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,177 | 32,154
Estimated Fair Value | 4,956,876 | 4,725,686
Non - credit other - than - temporary Impairments | 27,439 | 7,773
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,556 | 64,378
Gross Unrealized Gains | 416 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 111
Estimated Fair Value | 64,972 | 64,267
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 | 420,465 | 523,019
Gross Unrealized Gains | 5,217 | 9,769
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6,681 | 6,996
Estimated Fair Value | 419,001 | 525,792
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 | 1,593,457 | 1,382,195
Gross Unrealized Gains | 11,739 | 12,815
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,618 | 9,416
Estimated Fair Value | 1,587,578 | 1,385,594
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 | 104,226 | 126,041
Gross Unrealized Gains | 1,710 | 3,060
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2,282 | 1,344
Estimated Fair Value | 103,654 | 127,757
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 | 30,410 | 42,328
Gross Unrealized Gains | 672 | 1,078
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 219 | 427
Estimated Fair Value | 30,863 | 42,979
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 | 64,556 | 64,378
Gross Unrealized Gains | 416 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 111
Estimated Fair Value | 64,972 | 64,267
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 | 4,283 | 29,524
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 38
Estimated Fair Value | 4,281 | 29,486
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 | 1,797,616 | 1,557,059
Gross Unrealized Gains | 116,052 | 167,396
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 42,185 | 13,500
Estimated Fair Value | 1,871,483 | 1,710,955
Non - credit other - than - temporary Impairments | 27,439 | 7,773
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 82,734 | 21,346
Gross Unrealized Gains | 54 | 50
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 876 | 274
Estimated Fair Value | 81,912 | 21,122
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 | 836,309 | 833,366
Gross Unrealized Gains | 23,109 | 48,794
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,314 | 159
Estimated Fair Value | 858,104 | 882,001
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 | 5,234,284 | 4,874,947
Gross Unrealized Gains | 158,553 | 242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,181 | 32,158
Estimated Fair Value | 5,321,656 | 5,085,751
Non - credit other - than - temporary Impairments | $ 27,439 | $ 7,773

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