Stockholders' Equity and Noncontrolling Interests - Summary of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Weighted average shares outstanding	26,881,700			26,886,498			26,883,025	26,886,299
Incremental shares from RS awards and RSUs	6,472			6,515			6,313	37,241
Total shares for diluted calculations	26,888,172			26,893,013			26,889,338	26,923,540
Net income attributable to American National (in thousands)	$ 92,184	$ 98,840	$ 258,217	$ 173,464	$ 84,139	$ 18,777	$ 449,241	$ 276,380
Basic earnings per share (in usd per share)	$ 3.43			$ 6.45			$ 16.71	$ 10.28
Diluted earnings per share (in usd per share)	$ 3.43			$ 6.44			$ 16.71	$ 10.26