Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 825,495	$ 411,509
Unrealized Loss, Less than Twelve Months	(64,997)	(5,846)
Fair Value, Twelve Months or Longer	287,804	384,305
Unrealized Loss, Twelve Months or Longer	(83,148)	(77,389)
Fair Value	1,113,299	795,814
Total	(148,145)	(83,235)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	684,971	389,012
Unrealized Loss, Less than Twelve Months	(47,235)	(5,565)
Fair Value, Twelve Months or Longer	27,150	91,533
Unrealized Loss, Twelve Months or Longer	(3,331)	(4,986)
Fair Value	712,121	480,545
Total	(50,566)	(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	1,968	21,044
Unrealized Loss, Twelve Months or Longer	(58)	(294)
Fair Value	1,968	22,299
Total	(58)	(296)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	21,170	66,774
Unrealized Loss, Less than Twelve Months	(478)	(626)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	21,170	66,774
Total	(478)	(626)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	4,690	6,496
Unrealized Loss, Less than Twelve Months	(17)	(396)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	4,690	6,496
Total	(17)	(396)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	611,381	314,487
Unrealized Loss, Less than Twelve Months	(41,098)	(4,541)
Fair Value, Twelve Months or Longer	25,174	70,489
Unrealized Loss, Twelve Months or Longer	(3,273)	(4,692)
Fair Value	636,555	384,976
Total	(44,371)	(9,233)
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	47,730	0
Unrealized Loss, Less than Twelve Months	(5,642)	0
Fair Value, Twelve Months or Longer	8	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	47,738	0
Total	(5,642)	0
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	205,161	117,389
Unrealized Loss, Less than Twelve Months	(10,765)	(1,733)
Fair Value, Twelve Months or Longer	5,594	7,183
Unrealized Loss, Twelve Months or Longer	(2,906)	(1,317)
Fair Value	210,755	124,572
Total	(13,671)	(3,050)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	8,400
Unrealized Loss, Less than Twelve Months	0	(166)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	8,400
Total	0	(166)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	299,831	52,312
Unrealized Loss, Less than Twelve Months	(23,252)	(1,058)
Fair Value, Twelve Months or Longer	1,736	1,833
Unrealized Loss, Twelve Months or Longer	(212)	(115)
Fair Value	301,567	54,145
Total	(23,464)	(1,173)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	106,389	136,386
Unrealized Loss, Less than Twelve Months	(7,081)	(1,584)
Fair Value, Twelve Months or Longer	17,844	61,473
Unrealized Loss, Twelve Months or Longer	(155)	(3,260)
Fair Value	124,233	197,859
Total	(7,236)	(4,844)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	140,524	22,497
Unrealized Loss, Less than Twelve Months	(17,762)	(281)
Fair Value, Twelve Months or Longer	260,654	292,772
Unrealized Loss, Twelve Months or Longer	(79,817)	(72,403)
Fair Value	401,178	315,269
Total	(97,579)	(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	140,524	22,497
Unrealized Loss, Less than Twelve Months	(17,762)	(281)
Fair Value, Twelve Months or Longer	238,260	266,077
Unrealized Loss, Twelve Months or Longer	(68,110)	(64,848)
Fair Value	378,784	288,574
Total	(85,872)	(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	10,080	12,816
Unrealized Loss, Twelve Months or Longer	(9,920)	(7,184)
Fair Value	10,080	12,816
Total	(9,920)	(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	12,314	13,879
Unrealized Loss, Twelve Months or Longer	(1,787)	(371)
Fair Value	12,314	13,879
Total	(1,787)	(371)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	33,256	0
Unrealized Loss, Less than Twelve Months	(6,676)	0
Fair Value, Twelve Months or Longer	99,492	113,481
Unrealized Loss, Twelve Months or Longer	(33,217)	(19,257)
Fair Value	132,748	113,481
Total	(39,893)	(19,257)
Below Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	11,387	7,529
Unrealized Loss, Less than Twelve Months	(246)	(135)
Fair Value, Twelve Months or Longer	14,307	14,985
Unrealized Loss, Twelve Months or Longer	(1,370)	(1,264)
Fair Value	25,694	22,514
Total	(1,616)	(1,399)
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	62,252	14,968
Unrealized Loss, Less than Twelve Months	(8,099)	(146)
Fair Value, Twelve Months or Longer	55,653	69,956
Unrealized Loss, Twelve Months or Longer	(22,298)	(32,406)
Fair Value	117,905	84,924
Total	(30,397)	(32,552)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	33,629	0
Unrealized Loss, Less than Twelve Months	(2,741)	0
Fair Value, Twelve Months or Longer	68,808	67,655
Unrealized Loss, Twelve Months or Longer	(11,225)	(11,921)
Fair Value	102,437	67,655
Total	$ (13,966)	$ (11,921)