Investments - Summary of Fixed Maturities Available for Sale by Component (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 16,209,723	$ 15,753,471
Fair Value	18,797,674	16,297,932
Fixed maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	16,209,723	15,753,471
Gross Unrealized Gains	2,682,800	972,840
Gross Unrealized Losses	(94,849)	(428,379)
Fair Value	$ 18,797,674	$ 16,297,932
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]
Cost or Amortized Cost	$ 393,186	$ 390,351
Gross Unrealized Gains	55,730	5,104
Gross Unrealized Losses	(291)	(2,787)
Fair Value	$ 448,625	$ 392,668
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]
Cost or Amortized Cost	$ 1,459,986	$ 1,354,810
Gross Unrealized Gains	174,070	83,600
Gross Unrealized Losses	(242)	(1,750)
Fair Value	$ 1,633,814	$ 1,436,660
Percentage of Fixed Maturities at Fair Value	9.00%	9.00%
Fixed maturities available for sale \| Foreign governments
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 24,428	$ 19,006
Gross Unrealized Gains	2,442	1,810
Gross Unrealized Losses	(98)	0
Fair Value	$ 26,772	$ 20,816
Percentage of Fixed Maturities at Fair Value	0.00%	0.00%
Fixed maturities available for sale \| Corporates
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 14,130,749	$ 13,784,681
Gross Unrealized Gains	2,416,316	858,125
Gross Unrealized Losses	(87,013)	(416,794)
Fair Value	$ 16,460,052	$ 14,226,012
Percentage of Fixed Maturities at Fair Value	88.00%	87.00%
Fixed maturities available for sale \| Corporates \| Financial
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 4,031,919	$ 3,759,768
Gross Unrealized Gains	721,475	262,875
Gross Unrealized Losses	(27,772)	(87,515)
Fair Value	$ 4,725,622	$ 3,935,128
Percentage of Fixed Maturities at Fair Value	25.00%	24.00%
Fixed maturities available for sale \| Corporates \| Utilities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 1,939,618	$ 1,989,506
Gross Unrealized Gains	454,518	217,846
Gross Unrealized Losses	(1,640)	(24,399)
Fair Value	$ 2,392,496	$ 2,182,953
Percentage of Fixed Maturities at Fair Value	13.00%	13.00%
Fixed maturities available for sale \| Corporates \| Energy
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 1,658,861	$ 1,652,700
Gross Unrealized Gains	269,339	93,880
Gross Unrealized Losses	(35,836)	(62,371)
Fair Value	$ 1,892,364	$ 1,684,209
Percentage of Fixed Maturities at Fair Value	10.00%	10.00%
Fixed maturities available for sale \| Corporates \| Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 6,500,351	$ 6,382,707
Gross Unrealized Gains	970,984	283,524
Gross Unrealized Losses	(21,765)	(242,509)
Fair Value	$ 7,449,570	$ 6,423,722
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]
Cost or Amortized Cost	$ 56,815	$ 57,769
Gross Unrealized Gains	28,275	22,014
Gross Unrealized Losses	(6,929)	(6,414)
Fair Value	$ 78,161	$ 73,369
Percentage of Fixed Maturities at Fair Value	0.00%	1.00%
Fixed maturities available for sale \| Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 144,559	$ 146,854
Gross Unrealized Gains	5,967	2,187
Gross Unrealized Losses	(276)	(634)
Fair Value	$ 150,250	$ 148,407
Percentage of Fixed Maturities at Fair Value	1.00%	1.00%

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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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Amortized cost 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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