Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Premiums and other revenues	$ 1,057,273	$ 932,500	$ 956,677	$ 1,123,693	$ 518,700	$ 1,052,236	$ 952,071	$ 803,375	$ 4,070,143	$ 3,326,382	$ 3,411,000
Benefits, losses and expenses	835,864	848,134	852,773	838,995	682,868	868,969	850,796	782,591	3,375,766	3,185,224	3,082,654
Income (loss) before federal income tax and other items	221,409	84,366	103,904	284,698	(164,168)	183,267	101,275	20,784	694,377	141,158	328,346
Provision (benefit) for federal income taxes	53,617	22,475	21,948	67,377	(40,920)	19,219	21,957	1,189	165,417	1,445	(92,995)
Equity in earnings (losses) of unconsolidated affiliates	1,176	45,075	16,790	40,460	2,376	13,029	6,421	(545)	103,501	21,281	86,674
Other components of net periodic pension costs, net of tax	2,124	(1,023)	(871)	(914)	3,133	(1,236)	(1,677)	(792)	(684)	(572)	(12,408)
Net income (loss)	171,092	105,943	97,875	256,867	(117,739)	175,841	84,062	18,258	631,777	160,422	495,607
Less: Net income (loss) attributable to noncontrolling interest, net of tax	(30)	13,759	(965)	(1,350)	(354)	2,377	(77)	(519)	11,414	1,427	1,956
Net income (loss) attributable to American National	$ 171,122	$ 92,184	$ 98,840	$ 258,217	$ (117,385)	$ 173,464	$ 84,139	$ 18,777	$ 620,363	$ 158,995	$ 493,651
Earnings per share attributable to American National
Basic (in usd per share)	$ 6.37	$ 3.43	$ 3.68	$ 9.60	$ (4.37)	$ 6.45	$ 3.13	$ 0.70	$ 23.08	$ 5.91	$ 18.35
Diluted (in usd per share)	$ 6.37	$ 3.43	$ 3.67	$ 9.60	$ (4.37)	$ 6.44	$ 3.12	$ 0.70	$ 23.07	$ 5.91	$ 18.31