Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 411,509	$ 4,698,830
Unrealized Loss, Less than Twelve Months	(5,846)	(252,192)
Fair Value, Twelve Months or Longer	384,305	1,290,990
Unrealized Loss, Twelve Months or Longer	(77,389)	(176,187)
Fair Value	795,814	5,989,820
Total	(83,235)	(428,379)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	389,012	4,458,883
Unrealized Loss, Less than Twelve Months	(5,565)	(229,105)
Fair Value, Twelve Months or Longer	91,533	1,091,128
Unrealized Loss, Twelve Months or Longer	(4,986)	(106,067)
Fair Value	480,545	5,550,011
Total	(10,551)	(335,172)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,255	37,182
Unrealized Loss, Less than Twelve Months	(2)	(212)
Fair Value, Twelve Months or Longer	21,044	89,664
Unrealized Loss, Twelve Months or Longer	(294)	(2,575)
Fair Value	22,299	126,846
Total	(296)	(2,787)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	66,774	124,907
Unrealized Loss, Less than Twelve Months	(626)	(1,648)
Fair Value, Twelve Months or Longer	0	7,981
Unrealized Loss, Twelve Months or Longer	0	(102)
Fair Value	66,774	132,888
Total	(626)	(1,750)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,496	0
Unrealized Loss, Less than Twelve Months	(396)	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	6,496	0
Total	(396)	0
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	314,487	4,252,191
Unrealized Loss, Less than Twelve Months	(4,541)	(226,611)
Fair Value, Twelve Months or Longer	70,489	993,483
Unrealized Loss, Twelve Months or Longer	(4,692)	(103,390)
Fair Value	384,976	5,245,674
Total	(9,233)	(330,001)
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	44,603
Unrealized Loss, Less than Twelve Months	0	(634)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	44,603
Total	0	(634)
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	117,389	931,161
Unrealized Loss, Less than Twelve Months	(1,733)	(36,337)
Fair Value, Twelve Months or Longer	7,183	241,442
Unrealized Loss, Twelve Months or Longer	(1,317)	(21,572)
Fair Value	124,572	1,172,603
Total	(3,050)	(57,909)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,400	329,753
Unrealized Loss, Less than Twelve Months	(166)	(11,680)
Fair Value, Twelve Months or Longer	0	121,308
Unrealized Loss, Twelve Months or Longer	0	(9,442)
Fair Value	8,400	451,061
Total	(166)	(21,122)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	52,312	475,736
Unrealized Loss, Less than Twelve Months	(1,058)	(29,426)
Fair Value, Twelve Months or Longer	1,833	54,937
Unrealized Loss, Twelve Months or Longer	(115)	(9,382)
Fair Value	54,145	530,673
Total	(1,173)	(38,808)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	136,386	2,515,541
Unrealized Loss, Less than Twelve Months	(1,584)	(149,168)
Fair Value, Twelve Months or Longer	61,473	575,796
Unrealized Loss, Twelve Months or Longer	(3,260)	(62,994)
Fair Value	197,859	3,091,337
Total	(4,844)	(212,162)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	22,497	239,947
Unrealized Loss, Less than Twelve Months	(281)	(23,087)
Fair Value, Twelve Months or Longer	292,772	199,862
Unrealized Loss, Twelve Months or Longer	(72,403)	(70,120)
Fair Value	315,269	439,809
Total	(72,684)	(93,207)
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	22,497	239,947
Unrealized Loss, Less than Twelve Months	(281)	(23,087)
Fair Value, Twelve Months or Longer	266,077	186,276
Unrealized Loss, Twelve Months or Longer	(64,848)	(63,706)
Fair Value	288,574	426,223
Total	(65,129)	(86,793)
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	12,816	13,586
Unrealized Loss, Twelve Months or Longer	(7,184)	(6,414)
Fair Value	12,816	13,586
Total	(7,184)	(6,414)
Below Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0
Unrealized Loss, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	13,879
Unrealized Loss, Twelve Months or Longer	(371)
Fair Value	13,879
Total	(371)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	22,087
Unrealized Loss, Less than Twelve Months	0	(8,674)
Fair Value, Twelve Months or Longer	113,481	81,101
Unrealized Loss, Twelve Months or Longer	(19,257)	(20,932)
Fair Value	113,481	103,188
Total	(19,257)	(29,606)
Below Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	7,529	28,613
Unrealized Loss, Less than Twelve Months	(135)	(3,277)
Fair Value, Twelve Months or Longer	14,985	0
Unrealized Loss, Twelve Months or Longer	(1,264)	0
Fair Value	22,514	28,613
Total	(1,399)	(3,277)
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	14,968	42,874
Unrealized Loss, Less than Twelve Months	(146)	(3,901)
Fair Value, Twelve Months or Longer	69,956	36,122
Unrealized Loss, Twelve Months or Longer	(32,406)	(19,662)
Fair Value	84,924	78,996
Total	(32,552)	(23,563)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	146,373
Unrealized Loss, Less than Twelve Months	0	(7,235)
Fair Value, Twelve Months or Longer	67,655	69,053
Unrealized Loss, Twelve Months or Longer	(11,921)	(23,112)
Fair Value	67,655	215,426
Total	$ (11,921)	$ (30,347)