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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 | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 3,833,380 | $ 5,271,818
Gross Unrealized Gains | 194,408 | 145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,773 | 108,043
Estimated Fair Value | 3,993,015 | 5,309,161
Non - credit other - than - temporary Impairments | 7,306 | 23,832
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,756,924 | 2,914,145
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 393,516 | 557,476
Gross Unrealized Gains | 6 | 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 75 | 209
Estimated Fair Value | 393,447 | 557,270
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 3,355,223 | 4,614,623
Gross Unrealized Gains | 194,402 | 145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,698 | 107,834
Estimated Fair Value | 3,514,927 | 4,652,172
Non - credit other - than - temporary Impairments | 7,306 | 23,832
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 84,641 | 99,719
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 84,641 | 99,719
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 | 862,256 | 845,778
Gross Unrealized Gains | 38,667 | 3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,801 | 69,400
Estimated Fair Value | 899,122 | 779,834
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 | 1,354,633 | 858,774
Gross Unrealized Gains | 15,508 | 6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 19,354 | 5,471
Estimated Fair Value | 1,350,787 | 860,075
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 | 25,347 | 26,245
Gross Unrealized Gains | 269 | 409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 174 | 111
Estimated Fair Value | 25,442 | 26,543
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 | 83,579 | 86,900
Gross Unrealized Gains | 629 | 261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,795 | 1,753
Estimated Fair Value | 82,413 | 85,408
Non - credit other - than - temporary Impairments |  | 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 | 84,641 | 99,719
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 84,641 | 99,719
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 | 460,252 | 2,214,512
Gross Unrealized Gains | 59,441 | 67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 7,384 | 30,482
Estimated Fair Value | 512,309 | 2,251,333
Non - credit other - than - temporary Impairments | 7,306 | 23,832
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 65,255 | 50,754
Gross Unrealized Gains | 202 | 283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 65,457 | 51,037
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 | 503,901 | 531,660
Gross Unrealized Gains | 79,686 | 66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,190 | 617
Estimated Fair Value | 579,397 | 597,942
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 | 3,748,739 | 5,172,099
Gross Unrealized Gains | 194,408 | 145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,773 | 108,043
Estimated Fair Value | 3,908,374 | 5,209,442
Non - credit other - than - temporary Impairments | $ 7,306 | $ 23,832

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