Quarterly Financial Information (Unaudited) - Schedule of Quarterly Financial Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Jan. 31, 2014	Oct. 31, 2013	Jul. 31, 2013	Apr. 30, 2013	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2012
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]
Revenues	$ 59,491,000	$ 63,452,000	$ 57,864,000	$ 46,648,000	$ 57,840,000	$ 74,486,000	$ 82,619,000	$ 63,690,000	$ 227,455,000	$ 278,635,000	$ 141,850,000
Gross profit	20,687,000	23,876,000	21,257,000	13,028,000	14,019,000	13,136,000	12,879,000	10,101,000	78,848,000	50,135,000	23,102,000
Income from continuing operations	10,032,000	12,449,000	13,923,000	6,940,000	5,861,000	5,713,000	5,981,000	4,547,000	43,344,000	22,102,000	7,408,000
Net income	10,032,000	12,449,000	13,923,000	6,940,000	5,861,000	5,713,000	5,981,000	4,262,000	43,344,000	21,817,000	8,970,000
Net income attributable to the stockholders of Argan, Inc.	$ 9,164,000	$ 11,928,000	$ 12,623,000	$ 6,410,000	$ 6,561,000	$ 6,065,000	$ 6,201,000	$ 4,438,000	$ 40,125,000	$ 23,265,000	$ 9,272,000
Earnings per share
Basic	$ 0.64	$ 0.85	$ 0.90	$ 0.46	$ 0.47	$ 0.44	$ 0.45	$ 0.32	$ 2.85	$ 1.69	$ 0.68
Diluted	$ 0.63	$ 0.83	$ 0.89	$ 0.45	$ 0.46	$ 0.43	$ 0.45	$ 0.32	$ 2.78	$ 1.65	$ 0.67