Stockholders' Equity and Noncontrolling Interests - Summary of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Equity [Abstract]
Weighted average shares outstanding (in shares)	26,881,700	26,885,719
Incremental shares form RS award and RSUs (in shares)	9,975	6,185
Total shares for diluted calculations (in shares)	26,891,675	26,891,904
Net income (loss) attributable to American National (in thousands)	$ (220,444)	$ 258,217
Basic earnings per share (in usd per share)	$ (8.20)	$ 9.60
Diluted earnings per share (in usd per share)	$ (8.20)	$ 9.60