Investments - Summary of Fixed Maturities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 21,160,866	$ 21,213,509
Fixed maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,622,028	17,197,145
Allowance for Credit Losses	0	(3,346)
Gross Unrealized Gains	3,583,461	4,067,243
Gross Unrealized Losses	(44,623)	(47,533)
Fair Value	$ 21,160,866	$ 21,213,509
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	$ 380,193	$ 380,602
Allowance for Credit Losses	0	0
Gross Unrealized Gains	65,063	87,272
Gross Unrealized Losses	(140)	(43)
Fair Value	$ 445,116	$ 467,831
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	$ 2,068,436	$ 1,880,607
Allowance for Credit Losses	0	0
Gross Unrealized Gains	220,945	251,291
Gross Unrealized Losses	(8,433)	(315)
Fair Value	$ 2,280,948	$ 2,131,583
Percentage of Fixed Maturities at Fair Value	11.00%	10.00%
Fixed maturities available for sale \| Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 61,476	$ 52,913
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,363	2,635
Gross Unrealized Losses	(4,382)	(898)
Fair Value	$ 58,457	$ 54,650
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	$ 14,970,304	$ 14,691,277
Allowance for Credit Losses	0	(3,346)
Gross Unrealized Gains	3,263,971	3,698,971
Gross Unrealized Losses	(30,932)	(33,630)
Fair Value	$ 18,203,343	$ 18,353,272
Percentage of Fixed Maturities at Fair Value	86.00%	87.00%
Fixed maturities available for sale \| Corporates \| Financial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 4,563,734	$ 4,404,203
Allowance for Credit Losses	0	0
Gross Unrealized Gains	927,421	1,016,813
Gross Unrealized Losses	(12,474)	(24,221)
Fair Value	$ 5,478,681	$ 5,396,795
Percentage of Fixed Maturities at Fair Value	26.00%	26.00%
Fixed maturities available for sale \| Corporates \| Utilities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,935,489	$ 1,975,460
Allowance for Credit Losses	0	0
Gross Unrealized Gains	509,282	608,595
Gross Unrealized Losses	(988)	(108)
Fair Value	$ 2,443,783	$ 2,583,947
Percentage of Fixed Maturities at Fair Value	12.00%	12.00%
Fixed maturities available for sale \| Corporates \| Energy
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,603,163	$ 1,623,970
Allowance for Credit Losses	0	(3,346)
Gross Unrealized Gains	364,899	346,197
Gross Unrealized Losses	(2,131)	(3,083)
Fair Value	$ 1,965,931	$ 1,963,738
Percentage of Fixed Maturities at Fair Value	9.00%	9.00%
Fixed maturities available for sale \| Corporates \| Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,867,918	$ 6,687,644
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,462,369	1,727,366
Gross Unrealized Losses	(15,339)	(6,218)
Fair Value	$ 8,314,948	$ 8,408,792
Percentage of Fixed Maturities at Fair Value	39.00%	40.00%
Fixed maturities available for sale \| Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 36,088	$ 57,007
Allowance for Credit Losses	0	0
Gross Unrealized Gains	27,073	23,460
Gross Unrealized Losses	0	(8,869)
Fair Value	$ 63,161	$ 71,598
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	$ 105,531	$ 134,739
Allowance for Credit Losses	0	0
Gross Unrealized Gains	5,046	3,614
Gross Unrealized Losses	(736)	(3,778)
Fair Value	$ 109,841	$ 134,575
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 excluding accrued interest, 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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