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, 2014	$ 280,096	$ 1	$ 231,534	$ 48,542	$ 19
Beginning balance, shares at Jan. 31, 2014		124,791,545
Issuance of common stock upon exercise of stock options, value	5,813		5,813
Issuance of common stock upon exercise of stock options, shares		4,437,349
Vesting of early exercised stock options	377		377
Repurchase of unvested early exercised stock options		(16,667)
Issuance of common stock upon vesting of restricted stock units, value	(15)		(15)
Issuance of common stock upon vesting of restricted stock units, shares		115,339
Stock-based compensation expense	14,385		14,385
Issuance of common shares under Employee Stock Purchase Plan, value	5,951		5,951
Issuance of common shares under Employee Stock Purchase Plan, shares		350,059
Follow-on offering, net of issuance costs, value	34,495		34,495
Follow-on offering, net of issuance costs, shares		1,390,000
Excess tax benefits from employee stock plans	25,341		25,341
Other comprehensive income (loss)	7				7
Net income	40,383			40,383
Ending balance, value at Jan. 31, 2015	406,833	$ 1	317,881	88,925	26
Ending 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	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	40,903		40,903
Excess tax benefits from employee stock plans	25,628		25,628
Other comprehensive income (loss)	(61)				(61)
Net income	68,804			68,804
Ending balance, value at Jan. 31, 2017	$ 652,978	$ 1	$ 440,677	$ 212,189	$ 111
Ending balance, shares at Jan. 31, 2017		137,886,619