Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 3,089,195	$ 411,509
Unrealized Loss, Less than Twelve Months	(341,729)	(5,846)
Fair Value, Twelve Months or Longer	257,021	384,305
Unrealized Loss, Twelve Months or Longer	(110,382)	(77,389)
Fair Value	3,346,216	795,814
Total	(452,111)	(83,235)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,861,039	389,012
Unrealized Loss, Less than Twelve Months	(293,621)	(5,565)
Fair Value, Twelve Months or Longer	35,389	91,533
Unrealized Loss, Twelve Months or Longer	(6,905)	(4,986)
Fair Value	2,896,428	480,545
Total	(300,526)	(10,551)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	1,255
Unrealized Loss, Less than Twelve Months	0	(2)
Fair Value, Twelve Months or Longer	1,896	21,044
Unrealized Loss, Twelve Months or Longer	(124)	(294)
Fair Value	1,896	22,299
Total	(124)	(296)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	133,114	66,774
Unrealized Loss, Less than Twelve Months	(4,411)	(626)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	133,114	66,774
Total	(4,411)	(626)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	5,751	6,496
Unrealized Loss, Less than Twelve Months	(196)	(396)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	5,751	6,496
Total	(196)	(396)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,642,770	314,487
Unrealized Loss, Less than Twelve Months	(283,022)	(4,541)
Fair Value, Twelve Months or Longer	33,485	70,489
Unrealized Loss, Twelve Months or Longer	(6,781)	(4,692)
Fair Value	2,676,255	384,976
Total	(289,803)	(9,233)
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	79,404	0
Unrealized Loss, Less than Twelve Months	(5,992)	0
Fair Value, Twelve Months or Longer	8	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	79,412	0
Total	(5,992)	0
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	662,300	117,389
Unrealized Loss, Less than Twelve Months	(43,549)	(1,733)
Fair Value, Twelve Months or Longer	5,105	7,183
Unrealized Loss, Twelve Months or Longer	(3,395)	(1,317)
Fair Value	667,405	124,572
Total	(46,944)	(3,050)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	158,406	8,400
Unrealized Loss, Less than Twelve Months	(3,790)	(166)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	158,406	8,400
Total	(3,790)	(166)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	815,807	52,312
Unrealized Loss, Less than Twelve Months	(172,760)	(1,058)
Fair Value, Twelve Months or Longer	1,218	1,833
Unrealized Loss, Twelve Months or Longer	(730)	(115)
Fair Value	817,025	54,145
Total	(173,490)	(1,173)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,006,257	136,386
Unrealized Loss, Less than Twelve Months	(62,923)	(1,584)
Fair Value, Twelve Months or Longer	27,162	61,473
Unrealized Loss, Twelve Months or Longer	(2,656)	(3,260)
Fair Value	1,033,419	197,859
Total	(65,579)	(4,844)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	228,156	22,497
Unrealized Loss, Less than Twelve Months	(48,108)	(281)
Fair Value, Twelve Months or Longer	221,632	292,772
Unrealized Loss, Twelve Months or Longer	(103,477)	(72,403)
Fair Value	449,788	315,269
Total	(151,585)	(72,684)
Below Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Total	0	0
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	228,156	22,497
Unrealized Loss, Less than Twelve Months	(48,108)	(281)
Fair Value, Twelve Months or Longer	199,020	266,077
Unrealized Loss, Twelve Months or Longer	(91,914)	(64,848)
Fair Value	427,176	288,574
Total	(140,022)	(65,129)
Below Investment Grade | Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	9,876	12,816
Unrealized Loss, Twelve Months or Longer	(10,124)	(7,184)
Fair Value	9,876	12,816
Total	(10,124)	(7,184)
Below Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	12,736	13,879
Unrealized Loss, Twelve Months or Longer	(1,439)	(371)
Fair Value	12,736	13,879
Total	(1,439)	(371)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	19,232	0
Unrealized Loss, Less than Twelve Months	(3,397)	0
Fair Value, Twelve Months or Longer	91,734	113,481
Unrealized Loss, Twelve Months or Longer	(40,990)	(19,257)
Fair Value	110,966	113,481
Total	(44,387)	(19,257)
Below Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	27,081	7,529
Unrealized Loss, Less than Twelve Months	(3,405)	(135)
Fair Value, Twelve Months or Longer	12,976	14,985
Unrealized Loss, Twelve Months or Longer	(3,273)	(1,264)
Fair Value	40,057	22,514
Total	(6,678)	(1,399)
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	56,412	14,968
Unrealized Loss, Less than Twelve Months	(30,872)	(146)
Fair Value, Twelve Months or Longer	30,509	69,956
Unrealized Loss, Twelve Months or Longer	(31,391)	(32,406)
Fair Value	86,921	84,924
Total	(62,263)	(32,552)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	125,431	0
Unrealized Loss, Less than Twelve Months	(10,434)	0
Fair Value, Twelve Months or Longer	63,801	67,655
Unrealized Loss, Twelve Months or Longer	(16,260)	(11,921)
Fair Value	189,232	67,655
Total	$ (26,694)	$ (11,921)