Recently Issued Accounting Pronouncements - Summary of Financial Statement Amounts Previously Reported (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Prior Period Adjustments Reclassification [Line Items]
Investment in unconsolidated affiliates	$ 342,841		$ 342,841		$ 311,779
Deferred tax liabilities, net	226,894		226,894		287,175
Retained earnings	4,115,757		4,115,757		3,998,642
Provision for federal income taxes, current	30,037	$ 25,104	76,546	$ 59,337
Provision for federal income taxes, current	(1,307)	(1,786)	(3,792)	(3,432)
Provision for federal income taxes, deferred	(11,903)	3,431	2,488	9,759
Equity in earnings of unconsolidated affiliates	16,339	3,576	73,385	12,181
Net income attributable to American National	$ 42,013	66,939	$ 180,048	176,367
As Reported [Member]
Prior Period Adjustments Reclassification [Line Items]
Investment in unconsolidated affiliates					319,283
Deferred tax liabilities, net					290,112
Retained earnings					4,003,209
Provision for federal income taxes, current		23,639		55,690
Provision for federal income taxes, deferred		3,110		9,974
Equity in earnings of unconsolidated affiliates		2,735		10,405
Net income attributable to American National		67,884		178,023
Effect of Change [Member]
Prior Period Adjustments Reclassification [Line Items]
Provision for federal income taxes, current		1,465		3,647
Provision for federal income taxes, deferred		321		(215)
Equity in earnings of unconsolidated affiliates		841		1,776
Net income attributable to American National		$ (945)		$ (1,656)
Qualified Affordable Housing Project [Member]
Prior Period Adjustments Reclassification [Line Items]
Investment in unconsolidated affiliates					(7,504)
Deferred tax liabilities, net					(2,937)
Retained earnings					$ (4,567)