Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 278,339	$ 411,509
Unrealized Loss, Less than Twelve Months	(10,088)	(5,846)
Fair Value, Twelve Months or Longer	216,892	384,305
Unrealized Loss, Twelve Months or Longer	(37,445)	(77,389)
Fair Value	495,231	795,814
Total	(47,533)	(83,235)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	227,374	389,012
Unrealized Loss, Less than Twelve Months	(5,707)	(5,565)
Fair Value, Twelve Months or Longer	21,209	91,533
Unrealized Loss, Twelve Months or Longer	(2,274)	(4,986)
Fair Value	248,583	480,545
Total	(7,981)	(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	2,006	1,255
Unrealized Loss, Less than Twelve Months	(43)	(2)
Fair Value, Twelve Months or Longer	0	21,044
Unrealized Loss, Twelve Months or Longer	0	(294)
Fair Value	2,006	22,299
Total	(43)	(296)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	32,910	66,774
Unrealized Loss, Less than Twelve Months	(315)	(626)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	32,910	66,774
Total	(315)	(626)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	19,532	6,496
Unrealized Loss, Less than Twelve Months	(898)	(396)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	19,532	6,496
Total	(898)	(396)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	144,062	314,487
Unrealized Loss, Less than Twelve Months	(3,400)	(4,541)
Fair Value, Twelve Months or Longer	21,204	70,489
Unrealized Loss, Twelve Months or Longer	(2,274)	(4,692)
Fair Value	165,266	384,976
Total	(5,674)	(9,233)
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	28,864	0
Unrealized Loss, Less than Twelve Months	(1,051)	0
Fair Value, Twelve Months or Longer	5	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	28,869	0
Total	(1,051)	0
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	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Total	0	0
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	117,762	117,389
Unrealized Loss, Less than Twelve Months	(2,564)	(1,733)
Fair Value, Twelve Months or Longer	6,333	7,183
Unrealized Loss, Twelve Months or Longer	(2,168)	(1,317)
Fair Value	124,095	124,572
Total	(4,732)	(3,050)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,726	8,400
Unrealized Loss, Less than Twelve Months	(108)	(166)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	2,726	8,400
Total	(108)	(166)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,692	52,312
Unrealized Loss, Less than Twelve Months	(8)	(1,058)
Fair Value, Twelve Months or Longer	14,871	1,833
Unrealized Loss, Twelve Months or Longer	(106)	(115)
Fair Value	16,563	54,145
Total	(114)	(1,173)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	21,882	136,386
Unrealized Loss, Less than Twelve Months	(720)	(1,584)
Fair Value, Twelve Months or Longer	0	61,473
Unrealized Loss, Twelve Months or Longer	0	(3,260)
Fair Value	21,882	197,859
Total	(720)	(4,844)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	50,965	22,497
Unrealized Loss, Less than Twelve Months	(4,381)	(281)
Fair Value, Twelve Months or Longer	195,683	292,772
Unrealized Loss, Twelve Months or Longer	(35,171)	(72,403)
Fair Value	246,648	315,269
Total	(39,552)	(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	50,965	22,497
Unrealized Loss, Less than Twelve Months	(4,381)	(281)
Fair Value, Twelve Months or Longer	173,329	266,077
Unrealized Loss, Twelve Months or Longer	(23,575)	(64,848)
Fair Value	224,294	288,574
Total	(27,956)	(65,129)
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	11,223	13,879
Unrealized Loss, Twelve Months or Longer	(2,727)	(371)
Fair Value	11,223	13,879
Total	(2,727)	(371)
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	11,131	12,816
Unrealized Loss, Twelve Months or Longer	(8,869)	(7,184)
Fair Value	11,131	12,816
Total	(8,869)	(7,184)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,822	0
Unrealized Loss, Less than Twelve Months	(36)	0
Fair Value, Twelve Months or Longer	115,093	113,481
Unrealized Loss, Twelve Months or Longer	(19,453)	(19,257)
Fair Value	121,915	113,481
Total	(19,489)	(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	18,432	14,968
Unrealized Loss, Less than Twelve Months	(757)	(146)
Fair Value, Twelve Months or Longer	38,720	69,956
Unrealized Loss, Twelve Months or Longer	(2,212)	(32,406)
Fair Value	57,152	84,924
Total	(2,969)	(32,552)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	25,711	0
Unrealized Loss, Less than Twelve Months	(3,588)	0
Fair Value, Twelve Months or Longer	19,516	67,655
Unrealized Loss, Twelve Months or Longer	(1,910)	(11,921)
Fair Value	45,227	67,655
Total	$ (5,498)	$ (11,921)