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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, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 5,951,830 | $ 5,283,157
Gross Unrealized Gains | 222,697 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 63,773 | 104,206
Estimated Fair Value | 6,110,754 | 5,334,120
Non - credit other - than - temporary Impairments | 40,046 | 41,673
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,577,293 | 2,588,556
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 336,222 | 225,789
Gross Unrealized Gains | 0 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 336,222 | 225,789
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,550,887 | 4,992,756
Gross Unrealized Gains | 222,350 | 155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 63,773 | 104,148
Estimated Fair Value | 5,709,464 | 5,043,776
Non - credit other - than - temporary Impairments | 40,046 | 41,673
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,721 | 64,612
Gross Unrealized Gains | 347 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 65,068 | 64,555
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 | 367,363 | 424,048
Gross Unrealized Gains | 12,402 | 4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,113 | 8,188
Estimated Fair Value | 375,652 | 420,770
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 | 1,681,035 | 1,610,912
Gross Unrealized Gains | 50,637 | 7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 969 | 24,332
Estimated Fair Value | 1,730,703 | 1,593,669
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 | 69,374 | 96,638
Gross Unrealized Gains | 6,017 | 1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 1,823
Estimated Fair Value | 75,390 | 96,306
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 | 31,422 | 26,086
Gross Unrealized Gains | 2,948 | 789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 188
Estimated Fair Value | 34,369 | 26,687
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 | 64,721 | 64,612
Gross Unrealized Gains | 347 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 65,068 | 64,555
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 | 4,152 | 4,239
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 27
Estimated Fair Value | 4,148 | 4,212
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 | 2,221,276 | 1,942,285
Gross Unrealized Gains | 103,989 | 99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 54,722 | 64,617
Estimated Fair Value | 2,270,543 | 1,977,338
Non - credit other - than - temporary Impairments | 40,046 | 41,673
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 105,574 | 85,706
Gross Unrealized Gains | 232 | 42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,244 | 1,481
Estimated Fair Value | 104,562 | 84,267
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 | 1,070,691 | 802,842
Gross Unrealized Gains | 46,125 | 41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2,719 | 3,492
Estimated Fair Value | 1,114,097 | 840,527
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 | 5,887,109 | 5,218,545
Gross Unrealized Gains | 222,350 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 63,773 | 104,149
Estimated Fair Value | 6,045,686 | 5,269,565
Non - credit other - than - temporary Impairments | $ 40,046 | $ 41,673

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