Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Amortization of lease incentive obligation	(790)	(208)
Depreciation of property and equipment	5,581	2,155	784
Share-based compensation expense	15,596	12,143	6,847
Net (gain) loss on disposal of property and equipment	44	205
Deferred rent	973	3,594	565
Deferred income taxes		908	(698)
Tax benefit related to share-based compensation			(310)
Changes in operating assets and liabilities:
Accounts receivable	(59,000)	(1,000)
Prepaid expenses and other current assets	(10,469)	(5,502)	(1,337)
Other assets	(52)	(358)	(53)
Accounts payable	4,936	3,892	3,369
Accrued expenses and other current liabilities	5,864	4,550	2,296
Deferred revenue	160,585	(2,893)	11,408
Other non-current liabilities	523	823	864
Net cash used in operating activities	(22,862)	(83,671)	(31,925)
Cash flows from investing activities
Proceeds from the sale of property and equipment			4
Purchases of property and equipment, net of tenant improvement allowances	(9,938)	(18,887)	(5,567)
Net cash used in investing activities	(9,938)	(18,887)	(5,563)
Cash flows from financing activities
Costs associated with initial public offering			(1,075)
Proceeds from issuance of ordinary shares, net of offering costs	60,000	93,509	30,000
Payments on capital lease obligation	(16)	(62)	(62)
Proceeds from the exercise of share options	5,110	927	258
Net cash provided by financing activities	65,094	94,374	29,121
Effect of foreign exchange rates on cash	37	403	(14)
Net increase (decrease) in cash, cash equivalents and restricted cash	32,331	(7,781)	(8,381)
Cash, cash equivalents and restricted cash, beginning of period	146,113	153,894	162,275
Cash, cash equivalents and restricted cash, end of period	$ 178,444	$ 146,113	$ 153,894