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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 | Jun. 30, 2019 | Dec. 31, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 3,322,599 | $ 3,451,149
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 140,254 | 133,052
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 144,371 | 1,139,101
Non - credit other - than - temporary Impairments | 62 | 32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 942 | 27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 70,611 | 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,017 | 10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,959 | 38,195
Available-for-sale Securities, Current | 3,460,894 | 3,546,006
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 3,460,894 |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 772,214 | 430,405
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 87 | 23
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 4,695 | 115,374
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 97
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2 | 97
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 772,299 | 430,331
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,465,967 | 3,020,744
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 140,167 | 133,029
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 139,676 | 1,023,727
Non - credit other - than - temporary Impairments | 62 | 32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 940 | 27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 70,611 | 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,017 | 10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,957 | 38,098
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 2,604,177 | 3,115,675
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 312,476 | 882,631
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 31,106 | 14,364
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 21,593 | 537,904
Non - credit other - than - temporary Impairments | 0 | 5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 325 | 15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 25,981 | 28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 584 | 1,198
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 909 | 17,076
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 342,673 | 879,919
Fixed Income Securities [Member] | Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 1,365,426 | 1,288,882
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 35,670 | 6,444
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 22,520 | 306,506
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 78 | 8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 34,130 | 190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 359 | 8,570
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 437 | 17,204
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 1,400,659 | 1,278,122
Fixed Income Securities [Member] | Foreign Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 30,149 | 30,496
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 608 | 399
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 281 | 1,161
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 3,061 | 5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 14 | 60
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 14 | 61
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 30,743 | 30,834
Fixed Income Securities [Member] | US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 94,211 | 93,636
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2,634 | 1,371
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 2,833 | 5,643
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 21 | 135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 2,272 | 58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 18 | 478
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 39 | 613
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 96,806 | 94,394
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 211,081 | 221,825
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 41,241 | 37,575
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 4,500 | 34,852
Non - credit other - than - temporary Impairments | 62 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 62 | 793
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 62 | 793
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 252,260 | 258,607
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 136,547 | 133,075
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 47 | 8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 83,555 | 123,848
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 441 | 1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 441 | 1,727
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 136,153 | 131,356
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 316,077 | 370,199
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 28,861 | 72,868
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 4,394 | 13,813
Non - credit other - than - temporary Impairments | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 13 | 33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5,167 | 77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 42 | 591
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 55 | 624
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 344,883 | 442,443
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 3,238,181 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 140,254 |
Non - credit other - than - temporary Impairments | 62 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,959 |
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 3,376,476 |
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 84,418 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 |
Non - credit other - than - temporary Impairments | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 84,418 | $ 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 84,418 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 |
Non - credit other - than - temporary Impairments | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744) | 84,418 |
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 84,418 |

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- Definition Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 9A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284427-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580 + Details
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