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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 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 6,002,458 | $ 5,283,157
Gross Unrealized Gains | 236,300 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,206
Estimated Fair Value | 6,186,101 | 5,334,120
Non - credit other - than - temporary Impairments | 35,594 | 41,673
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 2,828,685 | 2,588,556
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 130,732 | 225,789
Gross Unrealized Gains | 0 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 130,732 | 225,789
Non - credit other - than - temporary Impairments | 0 | 0
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,806,949 | 4,992,756
Gross Unrealized Gains | 236,105 | 155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,148
Estimated Fair Value | 5,990,397 | 5,043,776
Non - credit other - than - temporary Impairments | 35,594 | 41,673
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 64,777 | 64,612
Gross Unrealized Gains | 195 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 64,972 | 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 | 389,573 | 424,048
Gross Unrealized Gains | 14,700 | 4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,751 | 8,188
Estimated Fair Value | 399,522 | 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 | 1,821,981 | 1,610,912
Gross Unrealized Gains | 52,436 | 7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,661 | 24,332
Estimated Fair Value | 1,872,756 | 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 | 53,431 | 96,638
Gross Unrealized Gains | 3,498 | 1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6 | 1,823
Estimated Fair Value | 56,923 | 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 | 30,638 | 26,086
Gross Unrealized Gains | 2,810 | 789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 23 | 188
Estimated Fair Value | 33,425 | 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 | 64,777 | 64,612
Gross Unrealized Gains | 195 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
Estimated Fair Value | 64,972 | 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 | 4,108 | 4,239
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 27
Estimated Fair Value | 4,106 | 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 | 2,323,156 | 1,942,285
Gross Unrealized Gains | 109,663 | 99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 45,288 | 64,617
Estimated Fair Value | 2,387,531 | 1,977,338
Non - credit other - than - temporary Impairments | 35,594 | 41,673
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 106,582 | 85,706
Gross Unrealized Gains | 521 | 42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 259 | 1,481
Estimated Fair Value | 106,844 | 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 | 1,077,480 | 802,842
Gross Unrealized Gains | 52,477 | 41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 667 | 3,492
Estimated Fair Value | 1,129,290 | 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 | 5,937,681 | 5,218,545
Gross Unrealized Gains | 236,105 | 155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,149
Estimated Fair Value | 6,121,129 | 5,269,565
Non - credit other - than - temporary Impairments | $ 35,594 | $ 41,673

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