Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 789,797	$ 411,509
Unrealized Loss, Less than Twelve Months	(47,474)	(5,846)
Fair Value, Twelve Months or Longer	261,966	384,305
Unrealized Loss, Twelve Months or Longer	(58,941)	(77,389)
Fair Value	1,051,763	795,814
Total	(106,415)	(83,235)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	590,528	389,012
Unrealized Loss, Less than Twelve Months	(20,273)	(5,565)
Fair Value, Twelve Months or Longer	21,440	91,533
Unrealized Loss, Twelve Months or Longer	(3,991)	(4,986)
Fair Value	611,968	480,545
Total	(24,264)	(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	0	21,044
Unrealized Loss, Twelve Months or Longer	0	(294)
Fair Value	0	22,299
Total	0	(296)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	142,060	66,774
Unrealized Loss, Less than Twelve Months	(3,887)	(626)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	142,060	66,774
Total	(3,887)	(626)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	9,514	6,496
Unrealized Loss, Less than Twelve Months	(80)	(396)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	9,514	6,496
Total	(80)	(396)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	409,210	314,487
Unrealized Loss, Less than Twelve Months	(12,116)	(4,541)
Fair Value, Twelve Months or Longer	21,433	70,489
Unrealized Loss, Twelve Months or Longer	(3,991)	(4,692)
Fair Value	430,643	384,976
Total	(16,107)	(9,233)
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	29,744	0
Unrealized Loss, Less than Twelve Months	(4,190)	0
Fair Value, Twelve Months or Longer	7	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	29,751	0
Total	(4,190)	0
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	246,528	117,389
Unrealized Loss, Less than Twelve Months	(5,404)	(1,733)
Fair Value, Twelve Months or Longer	5,897	7,183
Unrealized Loss, Twelve Months or Longer	(2,603)	(1,317)
Fair Value	252,425	124,572
Total	(8,007)	(3,050)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,557	8,400
Unrealized Loss, Less than Twelve Months	(27)	(166)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	2,557	8,400
Total	(27)	(166)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	82,548	52,312
Unrealized Loss, Less than Twelve Months	(4,161)	(1,058)
Fair Value, Twelve Months or Longer	15,536	1,833
Unrealized Loss, Twelve Months or Longer	(1,388)	(115)
Fair Value	98,084	54,145
Total	(5,549)	(1,173)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	77,577	136,386
Unrealized Loss, Less than Twelve Months	(2,524)	(1,584)
Fair Value, Twelve Months or Longer	0	61,473
Unrealized Loss, Twelve Months or Longer	0	(3,260)
Fair Value	77,577	197,859
Total	(2,524)	(4,844)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	199,269	22,497
Unrealized Loss, Less than Twelve Months	(27,201)	(281)
Fair Value, Twelve Months or Longer	240,526	292,772
Unrealized Loss, Twelve Months or Longer	(54,950)	(72,403)
Fair Value	439,795	315,269
Total	(82,151)	(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	199,269	22,497
Unrealized Loss, Less than Twelve Months	(27,201)	(281)
Fair Value, Twelve Months or Longer	218,001	266,077
Unrealized Loss, Twelve Months or Longer	(43,452)	(64,848)
Fair Value	417,270	288,574
Total	(70,653)	(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	10,186	12,816
Unrealized Loss, Twelve Months or Longer	(9,814)	(7,184)
Fair Value	10,186	12,816
Total	(9,814)	(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	12,339	13,879
Unrealized Loss, Twelve Months or Longer	(1,684)	(371)
Fair Value	12,339	13,879
Total	(1,684)	(371)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	34,614	0
Unrealized Loss, Less than Twelve Months	(5,332)	0
Fair Value, Twelve Months or Longer	102,402	113,481
Unrealized Loss, Twelve Months or Longer	(30,292)	(19,257)
Fair Value	137,016	113,481
Total	(35,624)	(19,257)
Below Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	7,529
Unrealized Loss, Less than Twelve Months	0	(135)
Fair Value, Twelve Months or Longer	0	14,985
Unrealized Loss, Twelve Months or Longer	0	(1,264)
Fair Value	0	22,514
Total	0	(1,399)
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	117,927	14,968
Unrealized Loss, Less than Twelve Months	(12,588)	(146)
Fair Value, Twelve Months or Longer	43,620	69,956
Unrealized Loss, Twelve Months or Longer	(6,252)	(32,406)
Fair Value	161,547	84,924
Total	(18,840)	(32,552)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	46,728	0
Unrealized Loss, Less than Twelve Months	(9,281)	0
Fair Value, Twelve Months or Longer	71,979	67,655
Unrealized Loss, Twelve Months or Longer	(6,908)	(11,921)
Fair Value	118,707	67,655
Total	$ (16,189)	$ (11,921)