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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 3,451,149 | $ 5,271,818
Gross Unrealized Gains | 133,052 | 145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 38,195 | 108,043
Available-for-sale Securities, Current | 3,546,006 | 5,309,161
Estimated Fair Value | 3,546,006 |
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 32 | 23,832
Below Investment Grade Securities and Non-Rated Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 17,838 | 100,503
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value | 660,063 | 1,855,694
Corporate Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 2,737,286 | 2,914,145
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 430,405 | 557,476
Gross Unrealized Gains | 23 | 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 97 | 209
Estimated Fair Value | 430,331 | 557,270
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,020,744 | 4,614,623
Gross Unrealized Gains | 133,029 | 145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 38,098 | 107,834
Estimated Fair Value | 3,115,675 | 4,652,172
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 32 | 23,832
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 882,631 | 845,778
Gross Unrealized Gains | 14,364 | 3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,076 | 69,400
Estimated Fair Value | 879,919 | 779,834
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 5 | 0
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,288,882 | 858,774
Gross Unrealized Gains | 6,444 | 6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,204 | 5,471
Estimated Fair Value | 1,278,122 | 860,075
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 30,496 | 26,245
Gross Unrealized Gains | 399 | 409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 61 | 111
Estimated Fair Value | 30,834 | 26,543
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 93,636 | 86,900
Gross Unrealized Gains | 1,371 | 261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 613 | 1,753
Estimated Fair Value | 94,394 | 85,408
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 221,825 | 2,214,512
Gross Unrealized Gains | 37,575 | 67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 793 | 30,482
Estimated Fair Value | 258,607 | 2,251,333
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 27 | 23,832
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 133,075 | 50,754
Gross Unrealized Gains | 8 | 283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,727 | 0
Estimated Fair Value | 131,356 | 51,037
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 370,199 | 531,660
Gross Unrealized Gains | 72,868 | 66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 624 | 617
Estimated Fair Value | 442,443 | 597,942
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 5,172,099
Gross Unrealized Gains |  | 145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 108,043
Estimated Fair Value |  | 5,209,442
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 23,832
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 0 | 99,719
Gross Unrealized Gains |  | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Estimated Fair Value | $ 0 | 99,719
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 99,719
Gross Unrealized Gains |  | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Estimated Fair Value |  | 99,719
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | $ 99,719

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