Investment in Securities - Credit Quality Indicators for Amortized Cost of Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	$ 7,102,110	$ 7,367,412
AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	45,540	27,787
AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	522,936	430,342
A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	3,289,605	3,120,290
BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	3,128,223	3,679,410
BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	115,806	109,583
U.S. treasury and government
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	12,284	7,733
U.S. treasury and government \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
U.S. treasury and government \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	12,284	7,733
U.S. treasury and government \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
U.S. treasury and government \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
U.S. treasury and government \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
U.S. states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	104,039	109,445
U.S. states and political subdivisions \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	14,364	25,831
U.S. states and political subdivisions \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	49,327	43,964
U.S. states and political subdivisions \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	9,188	34,893
U.S. states and political subdivisions \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	25,770	0
U.S. states and political subdivisions \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	5,390	4,757
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	14,369	3,851
Foreign governments \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Foreign governments \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	13,355	2,820
Foreign governments \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	1,014	1,031
Foreign governments \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Foreign governments \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	6,810,518	6,992,095
Corporate debt securities \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	31,176	1,956
Corporate debt securities \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	400,666	262,830
Corporate debt securities \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	3,212,688	2,976,571
Corporate debt securities \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	3,061,595	3,647,496
Corporate debt securities \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	104,393	103,242
Collateralized debt securities
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	112,409	139,709
Collateralized debt securities \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Collateralized debt securities \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Collateralized debt securities \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	66,715	107,795
Collateralized debt securities \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	40,858	31,914
Collateralized debt securities \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	4,836	0
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	48,491	114,579
Residential mortgage-backed securities \| AAA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Residential mortgage-backed securities \| AA
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	47,304	112,995
Residential mortgage-backed securities \| A
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Residential mortgage-backed securities \| BBB
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	0	0
Residential mortgage-backed securities \| BB and below
Schedule of Held-to-maturity Securities [Line Items]
Fixed maturity, bonds held-to-maturity	$ 1,187	$ 1,584

American National Group Inc 10-K