Consolidated Statements Stockholders' Equity - USD ($) $ in Thousands	Total	Class A & B Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balance, value at Jan. 31, 2015	$ 406,833	$ 1	$ 317,881	$ 88,925	$ 26
Beginning balance, shares at Jan. 31, 2015		131,067,625
Issuance of common stock upon exercise of stock options, value	5,898		5,898
Issuance of common stock upon exercise of stock options, shares		2,012,497
Issuance of common stock upon early exercise of stock options		22,084
Vesting of early exercised stock options	70		70
Repurchase of unvested early exercised stock options		(3,333)
Issuance of common stock upon vesting of restricted stock units, value	(6)		(6)
Issuance of common stock upon vesting of restricted stock units, shares		446,312
Stock-based compensation expense	24,321		24,321
Excess tax benefits from employee stock plans	13,527		13,527
Other comprehensive income (loss)	146				146
Net income	54,460			54,460
Ending balance, value at Jan. 31, 2016	505,249	$ 1	361,691	143,385	172
Ending balance, shares at Jan. 31, 2016		133,545,185
Issuance of common stock upon exercise of stock options, value	12,443		12,443
Issuance of common stock upon exercise of stock options, shares		3,369,356
Vesting of early exercised stock options	26		26
Issuance of common stock upon vesting of restricted stock units, value	(14)		(14)
Issuance of common stock upon vesting of restricted stock units, shares		972,078
Stock-based compensation expense	39,884		39,884
Excess tax benefits from employee stock plans	25,628		25,628
Other comprehensive income (loss)	(61)				(61)
Net income	69,499			69,499
Ending balance, value at Jan. 31, 2017	652,654	$ 1	439,658	212,884	111
Ending balance, shares at Jan. 31, 2017		137,886,619
Issuance of common stock upon exercise of stock options, value	$ 21,194		21,194
Issuance of common stock upon exercise of stock options, shares	2,935,962	2,935,962
Vesting of early exercised stock options	$ 1		1
Issuance of common stock upon vesting of restricted stock units, shares		1,246,815
Stock-based compensation expense	54,419		54,419
Other comprehensive income (loss)	1,293				1,293
Net income	141,966			141,966
Ending balance, value at Jan. 31, 2018	$ 871,527	$ 1	$ 515,272	$ 354,850	$ 1,404
Ending balance, shares at Jan. 31, 2018		142,069,396