Schedule for Fixed Income Securities at Amortized Cost, Gross Unrealized Gains and Losses and Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 7,716,912	$ 8,161,592
Gross Unrealized Gains	55,714	315,090
Gross Unrealized Losses	(368,352)	(61,721)
Fair Value	7,404,274	8,414,961
U.S. Treasury Securities and Obligations of U.S. Government Authority and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	305,336	416,639
Gross Unrealized Gains	5,423	5,322
Gross Unrealized Losses	(8,833)	(12,366)
Fair Value	301,926	409,595
Obligations of U.S. States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	747,872	741,343
Gross Unrealized Gains	21,407	37,908
Gross Unrealized Losses	(6,814)	(2,720)
Fair Value	762,465	776,531
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,476	24,242
Gross Unrealized Gains	8	234
Gross Unrealized Losses	(2,507)	(1,685)
Fair Value	19,977	22,791
All other corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,830,098	6,169,204
Gross Unrealized Gains	22,671	258,440
Gross Unrealized Losses	(328,363)	(37,898)
Fair Value	5,524,406	6,389,746
Asset-backed securities ("ABS")
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	404,935	478,329
Gross Unrealized Gains	422	5,289
Gross Unrealized Losses	(13,312)	(3,395)
Fair Value	392,045	480,223
Commercial mortgage-backed securities ("CMBS")
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	307,430	189,596
Gross Unrealized Gains	1,494	1,377
Gross Unrealized Losses	(7,957)	(3,148)
Fair Value	300,967	187,825
Residential mortgage-backed securities ("RMBS")
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	83,965	127,439
Gross Unrealized Gains	4,289	6,520
Gross Unrealized Losses	(566)	(509)
Fair Value	87,688	133,450
Redeemable Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,800	14,800
Fair Value	$ 14,800	$ 14,800

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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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss 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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