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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, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 6,041,185 | $ 5,283,157
Gross Unrealized Gains | 187,585 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 76,831 | 104,206
Estimated Fair Value | 6,151,939 | 5,334,120
Non - credit other - than - temporary Impairments | 43,928 | 41,673
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,622,724 | 2,588,556
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 436,759 | 225,789
Gross Unrealized Gains | 1 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 436,760 | 225,789
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,539,759 | 4,992,756
Gross Unrealized Gains | 187,333 | 155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 76,831 | 104,148
Estimated Fair Value | 5,650,261 | 5,043,776
Non - credit other - than - temporary Impairments | 43,928 | 41,673
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,667 | 64,612
Gross Unrealized Gains | 251 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 64,918 | 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 | 380,190 | 424,048
Gross Unrealized Gains | 10,609 | 4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,969 | 8,188
Estimated Fair Value | 385,830 | 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 | 1,626,276 | 1,610,912
Gross Unrealized Gains | 27,929 | 7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 7,901 | 24,332
Estimated Fair Value | 1,646,304 | 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 | 92,596 | 96,638
Gross Unrealized Gains | 4,106 | 1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 61 | 1,823
Estimated Fair Value | 96,641 | 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 | 32,185 | 26,086
Gross Unrealized Gains | 1,357 | 789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 93 | 188
Estimated Fair Value | 33,449 | 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 | 64,667 | 64,612
Gross Unrealized Gains | 251 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 64,918 | 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 | 4,195 | 4,239
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 9 | 27
Estimated Fair Value | 4,186 | 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 | 2,229,764 | 1,942,285
Gross Unrealized Gains | 103,842 | 99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 59,551 | 64,617
Estimated Fair Value | 2,274,055 | 1,977,338
Non - credit other - than - temporary Impairments | 43,928 | 41,673
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 100,949 | 85,706
Gross Unrealized Gains | 59 | 42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,667 | 1,481
Estimated Fair Value | 99,341 | 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 | 1,073,604 | 802,842
Gross Unrealized Gains | 39,431 | 41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2,580 | 3,492
Estimated Fair Value | 1,110,455 | 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 | 5,976,518 | 5,218,545
Gross Unrealized Gains | 187,334 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 76,831 | 104,149
Estimated Fair Value | 6,087,021 | 5,269,565
Non - credit other - than - temporary Impairments | $ 43,928 | $ 41,673

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