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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	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 3,472,642	$ 3,451,149
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,650	133,052
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	283,413	1,139,101
Non - credit other - than - temporary Impairments	123	32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,800	27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345	38,195
Available-for-sale Securities, Current	3,611,947	3,546,006
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,611,947
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	908,209	430,405
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	43	23
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	99,429	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	17	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	17	97
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	908,235	430,331
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,480,532	3,020,744
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,607	133,029
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	183,984	1,023,727
Non - credit other - than - temporary Impairments	123	32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,783	27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,328	38,098
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,619,811	3,115,675
Fixed Income Securities [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	406,315	882,631
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20,356	14,364
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	20,997	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	951	15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,865	28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	836	1,198
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,787	17,076
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	424,884	879,919
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,223,640	1,288,882
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14,312	6,444
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	39,639	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	550	8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	256,406	190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,858	8,570
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,408	17,204
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	1,233,544	1,278,122
Fixed Income Securities [Member] | Foreign Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	30,719	30,496
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	599	399
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	401	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	4	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,980	5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22	60
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26	61
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	31,292	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	99,808	93,636
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,185	1,371
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	12,793	5,643
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	360	135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	53,861	58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	204	478
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	564	613
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	100,429	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	216,904	221,825
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	39,504	37,575
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4,375	34,852
Non - credit other - than - temporary Impairments	123	27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	122	793
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	243	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	123	793
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	256,285	258,607
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	133,736	133,075
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	28	8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	101,939	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	787	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	787	1,727
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	132,977	131,356
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	369,410	370,199
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	71,623	72,868
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,840	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	9	33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	83,645	77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	624	591
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	633	624
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	440,400	442,443
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,388,741
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,650
Non - credit other - than - temporary Impairments	123
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,528,046
Collateral Pledged [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	83,901	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 $2,480,532 and $3,020,744)	83,901	$ 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	83,901
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 $2,480,532 and $3,020,744)	83,901
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 83,901

X

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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.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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