Revenue - Financial Statement Impact of ASC 606 (Income Statement) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Jul. 31, 2018	Jul. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ 148,222	$ 122,941	$ 288,729	$ 240,163
Operating expenses:
Sales and marketing	76,984	73,271	153,982	143,934
Loss from operations	(37,219)	(38,996)	(73,079)	(78,975)
Net loss	$ (38,085)	$ (39,285)	$ (74,722)	$ (79,371)
Net loss per common share, basic and diluted	$ (0.27)	$ (0.30)	$ (0.54)	$ (0.60)
Weighted-average shares used to compute net loss per share, basic and diluted	140,718	132,981	139,639	132,237
Balances without adoption of ASC Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ 150,941		$ 294,194
Operating expenses:
Sales and marketing	82,588		163,826
Loss from operations	(40,104)		(77,458)
Net loss	$ (40,970)		$ (79,101)
Net loss per common share, basic and diluted	$ (0.29)		$ (0.57)
Weighted-average shares used to compute net loss per share, basic and diluted	140,718		139,639
Effect of Change | Accounting Standards Update 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ (2,719)		$ (5,465)
Operating expenses:
Sales and marketing	(5,604)		(9,844)
Loss from operations	2,885		4,379
Net loss	$ 2,885		$ 4,379
Net loss per common share, basic and diluted	$ 0.02		$ 0.03
Weighted-average shares used to compute net loss per share, basic and diluted	140,718		139,639