Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 399,504	$ 4,698,830
Unrealized Loss	(7,430)	(252,192)
Fair Value	410,404	1,290,990
Unrealized Loss	(87,419)	(176,187)
Fair Value	809,908	5,989,820
Total	(94,849)	(428,379)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value	385,525	4,458,883
Unrealized Loss	(6,834)	(229,105)
Fair Value	156,966	1,091,128
Unrealized Loss	(12,378)	(106,067)
Fair Value	542,491	5,550,011
Total	(19,212)	(335,172)
Investment Grade \| U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,158	37,182
Unrealized Loss	(1)	(212)
Fair Value	21,089	89,664
Unrealized Loss	(290)	(2,575)
Fair Value	22,247	126,846
Total	(291)	(2,787)
Investment Grade \| States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value	19,654	124,907
Unrealized Loss	(242)	(1,648)
Fair Value	0	7,981
Unrealized Loss	0	(102)
Fair Value	19,654	132,888
Total	(242)	(1,750)
Investment Grade \| Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,400
Unrealized Loss	(98)
Fair Value	0
Unrealized Loss	0
Fair Value	6,400
Total	(98)
Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	358,313	4,252,191
Unrealized Loss	(6,493)	(226,611)
Fair Value	135,877	993,483
Unrealized Loss	(12,088)	(103,390)
Fair Value	494,190	5,245,674
Total	(18,581)	(330,001)
Investment Grade \| Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	44,603
Unrealized Loss	0	(634)
Fair Value	0	0
Unrealized Loss	0	0
Fair Value	0	44,603
Total	0	(634)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value	13,979	239,947
Unrealized Loss	(596)	(23,087)
Fair Value	253,438	199,862
Unrealized Loss	(75,041)	(70,120)
Fair Value	267,417	439,809
Total	(75,637)	(93,207)
Below Investment Grade \| States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value	0
Unrealized Loss	0
Fair Value	0
Unrealized Loss	0
Fair Value	0
Total	0
Below Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	13,979	239,947
Unrealized Loss	(596)	(23,087)
Fair Value	226,319	186,276
Unrealized Loss	(67,836)	(63,706)
Fair Value	240,298	426,223
Total	(68,432)	(86,793)
Below Investment Grade \| Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	0
Unrealized Loss	0	0
Fair Value	13,070	13,586
Unrealized Loss	(6,929)	(6,414)
Fair Value	13,070	13,586
Total	(6,929)	(6,414)
Below Investment Grade \| Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0
Unrealized Loss	0
Fair Value	14,049
Unrealized Loss	(276)
Fair Value	14,049
Total	(276)
Financial \| Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	157,535	931,161
Unrealized Loss	(2,335)	(36,337)
Fair Value	6,763	241,442
Unrealized Loss	(1,737)	(21,572)
Fair Value	164,298	1,172,603
Total	(4,072)	(57,909)
Financial \| Below Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	22,087
Unrealized Loss	0	(8,674)
Fair Value	109,052	81,101
Unrealized Loss	(23,700)	(20,932)
Fair Value	109,052	103,188
Total	(23,700)	(29,606)
Utilities \| Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	3,706	329,753
Unrealized Loss	(8)	(11,680)
Fair Value	0	121,308
Unrealized Loss	0	(9,442)
Fair Value	3,706	451,061
Total	(8)	(21,122)
Utilities \| Below Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,651	28,613
Unrealized Loss	(14)	(3,277)
Fair Value	14,631	0
Unrealized Loss	(1,618)	0
Fair Value	22,282	28,613
Total	(1,632)	(3,277)
Energy \| Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	71,563	475,736
Unrealized Loss	(2,439)	(29,426)
Fair Value	29,469	54,937
Unrealized Loss	(5,975)	(9,382)
Fair Value	101,032	530,673
Total	(8,414)	(38,808)
Energy \| Below Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,328	42,874
Unrealized Loss	(582)	(3,901)
Fair Value	38,839	36,122
Unrealized Loss	(26,840)	(19,662)
Fair Value	45,167	78,996
Total	(27,422)	(23,563)
Other corporate sectors \| Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	125,509	2,515,541
Unrealized Loss	(1,711)	(149,168)
Fair Value	99,645	575,796
Unrealized Loss	(4,376)	(62,994)
Fair Value	225,154	3,091,337
Total	(6,087)	(212,162)
Other corporate sectors \| Below Investment Grade \| Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	146,373
Unrealized Loss	0	(7,235)
Fair Value	63,797	69,053
Unrealized Loss	(15,678)	(23,112)
Fair Value	63,797	215,426
Total	$ (15,678)	$ (30,347)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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