Selected Quarterly Financial Data - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total premiums and other revenues	$ 518,700	$ 1,052,236	$ 952,071	$ 803,375	$ 924,360	$ 872,750	$ 834,093	$ 779,797	$ 3,326,382	$ 3,411,000	$ 3,227,951
Total benefits, losses and expenses	682,868	868,969	850,796	782,591	776,114	779,806	791,595	735,139	3,185,224	3,082,654	3,017,357
Income (loss) before federal income tax and other items	(164,168)	183,267	101,275	20,784	148,246	92,944	42,498	44,658	141,158	328,346	210,594
Total provision (benefit) for federal income taxes	(40,920)	19,219	21,957	1,189	(157,437)	37,183	13,524	13,735	1,445	(92,995)	77,706
Equity in earnings of unconsolidated affiliates	2,376	13,029	6,421	(545)	42,474	22,387	12,313	9,500	21,281	86,674	57,200
Net periodic pension costs net of tax	3,133	(1,236)	(1,677)	(792)	(4,043)	(1,545)	(5,588)	(1,232)	572	12,408	7,421
Net income (loss)	(117,739)	175,841	84,062	18,258	344,114	76,603	35,699	39,191	160,422	495,607	182,667
Net income (loss) attributable to noncontrolling interest, net of tax	(354)	2,377	(77)	(519)	(469)	3,334	(260)	(649)	1,427	1,956	1,664
Net income (loss) attributable to American National	$ (117,385)	$ 173,464	$ 84,139	$ 18,777	$ 344,583	$ 73,269	$ 35,959	$ 39,840	$ 158,995	$ 493,651	$ 181,003
Earnings per share attributable to American National
Basic (in usd per share)	$ (4.37)	$ 6.45	$ 3.13	$ 0.70	$ 12.81	$ 2.72	$ 1.34	$ 1.48	$ 5.91	$ 18.35	$ 6.73
Diluted (in usd per share)	$ (4.37)	$ 6.44	$ 3.12	$ 0.70	$ 12.78	$ 2.72	$ 1.33	$ 1.48	$ 5.91	$ 18.31	$ 6.71
Provisional tax benefit due to effect of reducing the statutory tax rate					$ 206,400				$ 206,400	$ 206,400
Net income excluding the impact of tax reform					$ 138,200
Basic earnings per share excluding the impact of tax reform					$ 5.14					$ 10.68
Diluted earnings per share excluding the impact of tax reform					$ 5.12					$ 10.65