Stockholders' Equity and Noncontrolling Interests - Summary of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity [Abstract]
Weighted average shares outstanding	26,881,700		26,883,276		26,883,699	26,886,196
Incremental shares from RS awards and RSUs	7,264		26,981		6,237	46,927
Total shares for diluted calculations	26,888,964		26,910,257		26,889,936	26,933,123
Net income attributable to American National (in thousands)	$ 98,840	$ 258,217	$ 84,139	$ 18,777	$ 357,057	$ 102,916
Basic earnings per share (in usd per share)	$ 3.68		$ 3.13		$ 13.28	$ 3.83
Diluted earnings per share (in usd per share)	$ 3.67		$ 3.12		$ 13.28	$ 3.82