Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 461,943	$ 278,339
Unrealized Loss, Less than Twelve Months	(9,688)	(10,088)
Fair Value, Twelve Months or Longer	248,093	216,892
Unrealized Loss, Twelve Months or Longer	(24,351)	(37,445)
Fair Value	710,036	495,231
Unrealized Loss	(34,039)	(47,533)
Increase (decrease) in gross unrealized losses	(13,500)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	445,548	227,374
Unrealized Loss, Less than Twelve Months	(9,405)	(5,707)
Fair Value, Twelve Months or Longer	124,933	21,209
Unrealized Loss, Twelve Months or Longer	(10,739)	(2,274)
Fair Value	570,481	248,583
Unrealized Loss	(20,144)	(7,981)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	118	2,006
Unrealized Loss, Less than Twelve Months	(1)	(43)
Fair Value, Twelve Months or Longer	3,867	0
Unrealized Loss, Twelve Months or Longer	(163)	0
Fair Value	3,985	2,006
Unrealized Loss	(164)	(43)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	141,310	32,910
Unrealized Loss, Less than Twelve Months	(2,824)	(315)
Fair Value, Twelve Months or Longer	2,436	0
Unrealized Loss, Twelve Months or Longer	(83)	0
Fair Value	143,746	32,910
Unrealized Loss	(2,907)	(315)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	12,567	19,532
Unrealized Loss, Less than Twelve Months	(561)	(898)
Fair Value, Twelve Months or Longer	23,144	0
Unrealized Loss, Twelve Months or Longer	(4,571)	0
Fair Value	35,711	19,532
Unrealized Loss	(5,132)	(898)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	281,064	144,062
Unrealized Loss, Less than Twelve Months	(6,003)	(3,400)
Fair Value, Twelve Months or Longer	95,485	21,204
Unrealized Loss, Twelve Months or Longer	(5,922)	(2,274)
Fair Value	376,549	165,266
Unrealized Loss	(11,925)	(5,674)
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
Unrealized Loss	0	0
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	10,489	28,864
Unrealized Loss, Less than Twelve Months	(16)	(1,051)
Fair Value, Twelve Months or Longer	1	5
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	10,490	28,869
Unrealized Loss	(16)	(1,051)
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	133,654	117,762
Unrealized Loss, Less than Twelve Months	(1,507)	(2,564)
Fair Value, Twelve Months or Longer	52,864	6,333
Unrealized Loss, Twelve Months or Longer	(1,932)	(2,168)
Fair Value	186,518	124,095
Unrealized Loss	(3,439)	(4,732)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	25,447	2,726
Unrealized Loss, Less than Twelve Months	(692)	(108)
Fair Value, Twelve Months or Longer	2,372	0
Unrealized Loss, Twelve Months or Longer	(320)	0
Fair Value	27,819	2,726
Unrealized Loss	(1,012)	(108)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,519	1,692
Unrealized Loss, Less than Twelve Months	(238)	(8)
Fair Value, Twelve Months or Longer	0	14,871
Unrealized Loss, Twelve Months or Longer	0	(106)
Fair Value	6,519	16,563
Unrealized Loss	(238)	(114)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	115,444	21,882
Unrealized Loss, Less than Twelve Months	(3,566)	(720)
Fair Value, Twelve Months or Longer	40,249	0
Unrealized Loss, Twelve Months or Longer	(3,670)	0
Fair Value	155,693	21,882
Unrealized Loss	(7,236)	(720)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	16,395	50,965
Unrealized Loss, Less than Twelve Months	(283)	(4,381)
Fair Value, Twelve Months or Longer	123,160	195,683
Unrealized Loss, Twelve Months or Longer	(13,612)	(35,171)
Fair Value	139,555	246,648
Unrealized Loss	(13,895)	(39,552)
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
Unrealized Loss	0	0
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	16,395	50,965
Unrealized Loss, Less than Twelve Months	(283)	(4,381)
Fair Value, Twelve Months or Longer	110,117	173,329
Unrealized Loss, Twelve Months or Longer	(13,198)	(23,575)
Fair Value	126,512	224,294
Unrealized Loss	(13,481)	(27,956)
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	0	11,131
Unrealized Loss, Twelve Months or Longer	0	(8,869)
Fair Value	0	11,131
Unrealized Loss	0	(8,869)
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	13,043	11,223
Unrealized Loss, Twelve Months or Longer	(414)	(2,727)
Fair Value	13,043	11,223
Unrealized Loss	(414)	(2,727)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	15,695	6,822
Unrealized Loss, Less than Twelve Months	(272)	(36)
Fair Value, Twelve Months or Longer	56,897	115,093
Unrealized Loss, Twelve Months or Longer	(5,638)	(19,453)
Fair Value	72,592	121,915
Unrealized Loss	(5,910)	(19,489)
Below Investment Grade | Utilities | Corporates
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
Unrealized Loss	0	0
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	18,432
Unrealized Loss, Less than Twelve Months	0	(757)
Fair Value, Twelve Months or Longer	26,639	38,720
Unrealized Loss, Twelve Months or Longer	(1,445)	(2,212)
Fair Value	26,639	57,152
Unrealized Loss	(1,445)	(2,969)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	700	25,711
Unrealized Loss, Less than Twelve Months	(11)	(3,588)
Fair Value, Twelve Months or Longer	26,581	19,516
Unrealized Loss, Twelve Months or Longer	(6,115)	(1,910)
Fair Value	27,281	45,227
Unrealized Loss	$ (6,126)	$ (5,498)