Stockholders' Equity and Noncontrolling Interests - Summary of Basic and Diluted Earnings Per Share (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
Equity [Abstract]
Weighted average shares outstanding (in shares)									26,882,691	26,886,357	26,896,926
Incremental shares from RS awards and RSUs									8,552	30,286	63,769
Total shares for diluted calculations (in shares)									26,891,243	26,916,643	26,960,695
Net income 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
Basic earnings per share (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 earnings per share (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
Provisional tax benefit due to effect of reducing the statutory tax rate									$ 206,400		$ 206,400
Net income excluding the impact of tax reform											$ 287,300
Basic earnings per share excluding the impact of tax reform											$ 10.68
Diluted earnings per share excluding the impact of tax reform											$ 10.65