Summary of Significant Accounting Policies - Summary of Income Statement Items After Adoption of New Accounting Standards (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		$ 430,492	$ 311,999	$ 208,768
Operating expenses:
Sales and marketing	[1]	220,742	166,987	114,052
Operating loss		(114,643)	(104,326)	(82,985)
Net loss		(110,638)	(103,799)	$ (84,052)
Pro Forma | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		430,165	312,844
Operating expenses:
Sales and marketing		211,918	161,653
Operating loss		(106,146)	(98,147)
Net loss		$ (102,141)	$ (97,620)

[1] Includes share-based compensation expense as follows as of December 31, 2017, 2016, and 2015:Cost of revenue: $9,040, $7,045, $4,541Research and development: $29,970, $27,083, $19,414Sales and marketing: $24,776, $23,043, $14,759General and administrative: $21,263, $16,608, $13,842