Investment in Securities - Investments in Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	$ 7,291,293			$ 7,367,412
Held-to-maturity securities, gross unrealized gains	486,773			639,648
Held-to-maturity securities, gross unrealized losses	(17,543)			(11,437)
Held-to-maturity securities, allowance for credit losses	(13,492)	$ (17,202)	$ (18,854)	(12,442)	$ (21,491)	$ (22,414)	$ (21,385)	$ (21,664)
Held-to-maturity securities, fair value	7,747,031			7,983,181
Available-for-sale securities, cost or amortized cost	8,207,687			7,073,142
Available-for-sale securities, gross unrealized gains	397,993			548,996
Available-for-sale securities, gross unrealized losses	(28,070)			(17,476)
Available-for-sale securities, allowance for credit losses	(4,529)	(5,158)	(8,716)	(7,482)	(7,638)	(4,115)	(12,865)	0
Available-for-sale securities, fair value	8,573,081			7,597,180
Investments in securities, cost or amortized cost	15,498,980			14,440,554
Investments in securities, gross unrealized gains	884,766			1,188,644
Investments in securities, gross unrealized losses	(45,613)			(28,913)
Investments in securities, allowance for credit losses	(18,021)			(19,924)
Investments in securities, fair value	16,320,112			15,580,361
U.S. treasury and government
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	12,320			7,733
Held-to-maturity securities, gross unrealized gains	0			11
Held-to-maturity securities, gross unrealized losses	(157)			0
Held-to-maturity securities, allowance for credit losses	0			0
Held-to-maturity securities, fair value	12,163			7,744
Available-for-sale securities, cost or amortized cost	25,880			28,766
Available-for-sale securities, gross unrealized gains	180			418
Available-for-sale securities, gross unrealized losses	(56)			(1)
Available-for-sale securities, allowance for credit losses	0	0	(3)	0
Available-for-sale securities, fair value	26,004			29,183
U.S. states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	108,155			109,445
Held-to-maturity securities, gross unrealized gains	2,226			4,101
Held-to-maturity securities, gross unrealized losses	(2,059)			(11)
Held-to-maturity securities, allowance for credit losses	0			0
Held-to-maturity securities, fair value	108,322			113,535
Available-for-sale securities, cost or amortized cost	1,029,345			1,066,627
Available-for-sale securities, gross unrealized gains	53,553			73,976
Available-for-sale securities, gross unrealized losses	(2,394)			(145)
Available-for-sale securities, allowance for credit losses	(16)	(8)	0	0
Available-for-sale securities, fair value	1,080,488			1,140,458
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	14,403			3,851
Held-to-maturity securities, gross unrealized gains	240			374
Held-to-maturity securities, gross unrealized losses	(110)			0
Held-to-maturity securities, allowance for credit losses	0			0	0	0	5	4
Held-to-maturity securities, fair value	14,533			4,225
Available-for-sale securities, cost or amortized cost	5,000			14,995
Available-for-sale securities, gross unrealized gains	980			1,393
Available-for-sale securities, gross unrealized losses	0			0
Available-for-sale securities, allowance for credit losses	0			0
Available-for-sale securities, fair value	5,980			16,388
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	6,995,994			6,992,095
Held-to-maturity securities, gross unrealized gains	479,215			623,233
Held-to-maturity securities, gross unrealized losses	(14,470)			(9,117)
Held-to-maturity securities, allowance for credit losses	(11,977)	(14,952)	(11,793)	(7,475)	(18,510)	(19,882)	(18,401)	(18,563)
Held-to-maturity securities, fair value	7,448,762			7,598,736
Available-for-sale securities, cost or amortized cost	6,905,556			5,887,756
Available-for-sale securities, gross unrealized gains	341,990			471,205
Available-for-sale securities, gross unrealized losses	(24,882)			(17,207)
Available-for-sale securities, allowance for credit losses	(3,569)	(4,489)	(8,008)	(7,275)	(7,437)	(3,824)	(12,499)	0
Available-for-sale securities, fair value	7,219,095			6,334,479
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	54,319			114,579
Held-to-maturity securities, gross unrealized gains	3,395			5,065
Held-to-maturity securities, gross unrealized losses	(448)			(1,464)
Held-to-maturity securities, allowance for credit losses	(506)	(526)	(542)	(452)	(35)	0	(3)	(137)
Held-to-maturity securities, fair value	56,760			117,728
Available-for-sale securities, cost or amortized cost	32,321			20,544
Available-for-sale securities, gross unrealized gains	602			964
Available-for-sale securities, gross unrealized losses	(84)			(29)
Available-for-sale securities, allowance for credit losses	(257)	(205)	(198)	(188)	(194)	(210)	(130)	0
Available-for-sale securities, fair value	32,582			21,291
Collateralized debt securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, cost or amortized cost	106,102			139,709
Held-to-maturity securities, gross unrealized gains	1,697			6,864
Held-to-maturity securities, gross unrealized losses	(299)			(845)
Held-to-maturity securities, allowance for credit losses	(1,009)	(1,724)	(6,519)	(4,515)	(2,946)	(2,532)	(2,986)	(2,968)
Held-to-maturity securities, fair value	106,491			141,213
Available-for-sale securities, cost or amortized cost	209,585			54,454
Available-for-sale securities, gross unrealized gains	688			1,040
Available-for-sale securities, gross unrealized losses	(654)			(94)
Available-for-sale securities, allowance for credit losses	(687)	$ (456)	$ (507)	(19)	$ (7)	$ (81)	$ (236)	$ 0
Available-for-sale securities, fair value	$ 208,932			$ 55,381

American National Group Inc 10-Q