Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 311,508	$ 280,921	$ 266,900	$ 244,752	$ 232,323	$ 224,731	$ 209,609	$ 195,547	$ 1,104,081	$ 862,210	$ 690,559
Gross profit	217,189	207,592	196,682	179,249	167,797	163,357	150,383	135,392	800,712	616,929	479,137
Operating income	60,394	80,800	73,856	71,169	62,998	63,094	52,818	43,956	286,219	222,866	157,929
Net income	$ 66,182	$ 82,245	$ 79,242	$ 73,449	$ 71,151	$ 64,085	$ 50,286	$ 44,310	$ 301,118	$ 229,832	$ 151,177
Net income per share:
Basic	$ 0.44	$ 0.56	$ 0.54	$ 0.50	$ 0.49	$ 0.44	$ 0.35	$ 0.31	$ 2.04	$ 1.59	$ 1.08
Diluted	$ 0.42	$ 0.52	$ 0.50	$ 0.47	$ 0.45	$ 0.41	$ 0.32	$ 0.29	$ 1.90	$ 1.47	$ 0.98