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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 | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 5,715,764 | $ 5,931,045
Gross Unrealized Gains | 208,763 | 198,505
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 68,565 | 79,642
Estimated Fair Value | 5,855,962 | 6,049,908
Non - credit other - than - temporary Impairments | 31,056 | 35,232
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,663,719 | 2,622,566
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 347,152 | 430,827
Gross Unrealized Gains | 12 | 5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 109 | 44
Estimated Fair Value | 347,055 | 430,788
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,303,723 | 5,435,385
Gross Unrealized Gains | 208,732 | 198,428
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 68,456 | 79,598
Estimated Fair Value | 5,443,999 | 5,554,215
Non - credit other - than - temporary Impairments | 31,056 | 35,232
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,889 | 64,833
Gross Unrealized Gains | 19 | 72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 64,908 | 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 | 458,363 | 376,064
Gross Unrealized Gains | 5,928 | 5,509
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 7,584 | 7,205
Estimated Fair Value | 456,707 | 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 | 1,769,546 | 1,803,136
Gross Unrealized Gains | 20,681 | 19,589
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 15,303 | 20,560
Estimated Fair Value | 1,774,924 | 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 | 28,652 | 41,932
Gross Unrealized Gains | 843 | 1,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 32 | 100
Estimated Fair Value | 29,463 | 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 | 41,074 | 33,732
Gross Unrealized Gains | 1,042 | 2,551
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 107 | 97
Estimated Fair Value | 42,009 | 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 | 64,889 | 64,833
Gross Unrealized Gains | 19 | 72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 64,908 | 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 | 4,020 | 4,063
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 3
Estimated Fair Value | 4,018 | 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 | 2,240,984 | 2,284,425
Gross Unrealized Gains | 110,762 | 110,955
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 37,259 | 43,785
Estimated Fair Value | 2,314,487 | 2,351,595
Non - credit other - than - temporary Impairments | 31,056 | 35,232
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 152,507 | 113,650
Gross Unrealized Gains | 601 | 493
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 220
Estimated Fair Value | 153,104 | 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 | 608,577 | 778,383
Gross Unrealized Gains | 68,875 | 58,028
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 8,165 | 7,628
Estimated Fair Value | 669,287 | 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 | 5,650,875 | 5,866,212
Gross Unrealized Gains | 208,744 | 198,433
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 68,565 | 79,642
Estimated Fair Value | 5,791,054 | 5,985,003
Non - credit other - than - temporary Impairments | $ 31,056 | $ 35,232

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