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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, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 5,734,282 | $ 5,931,045
Gross Unrealized Gains | 214,955 | 198,505
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 62,108 | 79,642
Estimated Fair Value | 5,887,129 | 6,049,908
Non - credit other - than - temporary Impairments | 27,624 | 35,232
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,722,316 | 2,622,566
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 504,093 | 430,827
Gross Unrealized Gains | 14 | 5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 101 | 44
Estimated Fair Value | 504,006 | 430,788
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,165,245 | 5,435,385
Gross Unrealized Gains | 214,941 | 198,428
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 61,991 | 79,598
Estimated Fair Value | 5,318,195 | 5,554,215
Non - credit other - than - temporary Impairments | 27,624 | 35,232
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,944 | 64,833
Gross Unrealized Gains | 0 | 72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 0
Estimated Fair Value | 64,928 | 64,905
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 | 662,514 | 376,064
Gross Unrealized Gains | 10,924 | 5,509
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 11,932 | 7,205
Estimated Fair Value | 661,506 | 374,368
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 | 1,487,806 | 1,803,136
Gross Unrealized Gains | 21,651 | 19,589
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 9,044 | 20,560
Estimated Fair Value | 1,500,413 | 1,802,165
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 | 29,201 | 41,932
Gross Unrealized Gains | 639 | 1,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 94 | 100
Estimated Fair Value | 29,746 | 43,135
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 | 41,625 | 33,732
Gross Unrealized Gains | 770 | 2,551
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 762 | 97
Estimated Fair Value | 41,633 | 36,186
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 | 64,944 | 64,833
Gross Unrealized Gains | 0 | 72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 0
Estimated Fair Value | 64,928 | 64,905
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 | 0 | 4,063
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 3
Estimated Fair Value | 0 | 4,060
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 | 2,220,640 | 2,284,425
Gross Unrealized Gains | 111,072 | 110,955
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 32,184 | 43,785
Estimated Fair Value | 2,299,528 | 2,351,595
Non - credit other - than - temporary Impairments | 27,624 | 35,232
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 124,913 | 113,650
Gross Unrealized Gains | 195 | 493
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 220
Estimated Fair Value | 125,106 | 113,923
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 | 598,546 | 778,383
Gross Unrealized Gains | 69,690 | 58,028
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 7,973 | 7,628
Estimated Fair Value | 660,263 | 828,783
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 | 5,669,338 | 5,866,212
Gross Unrealized Gains | 214,955 | 198,433
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 62,092 | 79,642
Estimated Fair Value | 5,822,201 | 5,985,003
Non - credit other - than - temporary Impairments | $ 27,624 | $ 35,232

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