Investments - Summary of Fixed Maturities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 17,197,145	$ 16,415,776
Allowance for Credit Losses	(3,346)	0
Fair Value	21,213,509	18,907,147
Corporates
Debt Securities, Available-for-sale [Line Items]
Allowance for Credit Losses	(3,346)	0	$ 0
Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value		74,104
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value		148,519
Fixed maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,197,145	16,415,776
Allowance for Credit Losses	(3,346)
Gross Unrealized Gains	4,067,243	2,574,606
Gross Unrealized Losses	(47,533)	(83,235)
Fair Value	$ 21,213,509	$ 18,907,147
Percentage of Fixed Maturities at Fair Value	100.00%	100.00%
Fixed maturities available for sale \| U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 380,602	$ 396,079
Allowance for Credit Losses	0
Gross Unrealized Gains	87,272	41,737
Gross Unrealized Losses	(43)	(296)
Fair Value	$ 467,831	$ 437,520
Percentage of Fixed Maturities at Fair Value	2.00%	2.00%
Fixed maturities available for sale \| States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,880,607	$ 1,559,736
Allowance for Credit Losses	0
Gross Unrealized Gains	251,291	158,546
Gross Unrealized Losses	(315)	(626)
Fair Value	$ 2,131,583	$ 1,717,656
Percentage of Fixed Maturities at Fair Value	10.00%	9.00%
Fixed maturities available for sale \| Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 52,913	$ 25,874
Allowance for Credit Losses	0
Gross Unrealized Gains	2,635	2,073
Gross Unrealized Losses	(898)	(396)
Fair Value	$ 54,650	$ 27,551
Percentage of Fixed Maturities at Fair Value	0.00%	0.00%
Fixed maturities available for sale \| Corporates
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 14,691,277	$ 14,233,301
Allowance for Credit Losses	(3,346)
Gross Unrealized Gains	3,698,971	2,342,858
Gross Unrealized Losses	(33,630)	(74,362)
Fair Value	$ 18,353,272	$ 16,501,797
Percentage of Fixed Maturities at Fair Value	87.00%	88.00%
Fixed maturities available for sale \| Corporates \| Financial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 4,404,203	$ 4,101,917
Allowance for Credit Losses	0
Gross Unrealized Gains	1,016,813	701,196
Gross Unrealized Losses	(24,221)	(22,307)
Fair Value	$ 5,396,795	$ 4,780,806
Percentage of Fixed Maturities at Fair Value	26.00%	25.00%
Fixed maturities available for sale \| Corporates \| Utilities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,975,460	$ 1,937,738
Allowance for Credit Losses	0
Gross Unrealized Gains	608,595	416,114
Gross Unrealized Losses	(108)	(1,565)
Fair Value	$ 2,583,947	$ 2,352,287
Percentage of Fixed Maturities at Fair Value	12.00%	13.00%
Fixed maturities available for sale \| Corporates \| Energy
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,623,970	$ 1,678,969
Allowance for Credit Losses	(3,346)
Gross Unrealized Gains	346,197	269,640
Gross Unrealized Losses	(3,083)	(33,725)
Fair Value	$ 1,963,738	$ 1,914,884
Percentage of Fixed Maturities at Fair Value	9.00%	10.00%
Fixed maturities available for sale \| Corporates \| Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,687,644	$ 6,514,677
Allowance for Credit Losses	0
Gross Unrealized Gains	1,727,366	955,908
Gross Unrealized Losses	(6,218)	(16,765)
Fair Value	$ 8,408,792	$ 7,453,820
Percentage of Fixed Maturities at Fair Value	40.00%	40.00%
Fixed maturities available for sale \| Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 57,007	$ 56,990
Allowance for Credit Losses	0
Gross Unrealized Gains	23,460	24,298
Gross Unrealized Losses	(8,869)	(7,184)
Fair Value	$ 71,598	$ 74,104
Percentage of Fixed Maturities at Fair Value	0.00%	0.00%
Fixed maturities available for sale \| Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 134,739	$ 143,796
Allowance for Credit Losses	0
Gross Unrealized Gains	3,614	5,094
Gross Unrealized Losses	(3,778)	(371)
Fair Value	$ 134,575	$ 148,519
Percentage of Fixed Maturities at Fair Value	1.00%	1.00%

Percentage of fixed maturities by component at fair value.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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