CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities
Net loss	$ (35,979)	$ (20,499)
Adjustment to reconcile net loss to net cash from operating activities
Depreciation and amortization	1,471	76
Stock-based compensation	16,058	7,241
Non-cash interest expense on convertible notes		6,113
Deferred rent	668	36
Loss on disposal of assets		(1)
Gain on adjustment of contingent consideration	(580)
Changes in operating assets and liabilities
Deferred revenue	54,078
Prepaid expenses and other current assets	(6,141)	(557)
Other assets	(9,098)	(99)
Accounts payable	2,127	1,199
Accrued expenses and other current liabilities	1,711	(67)
Due to related party	78
Net cash provided by (used in) operating activities	24,393	(6,558)
Cash flows from investing activities
Purchases of marketable securities	(48,526)
Sales and maturities of marketable securities	76,572
Amounts used to secure facilities	(1,540)
Purchase of property and equipment	(9,456)	(1,002)
Net cash paid related to acquisition of T-Cell Factory, B.V.	(14,690)
Net cash provided by (used in) investing activities	2,360	(1,002)
Cash flows from financing activities
Principal payments on capital lease obligations	(16)
Initial public offering costs		(10,943)
Proceeds from issuance of common stock	26,664	146,625
Proceeds from exercise of stock options	694	2,918
Proceeds from issuance of convertible notes		50,000
Net cash provided by financing activities	27,341	188,600
Effect of exchange rate changes on cash	(155)
Net change in cash and cash equivalents	53,939	181,040
Cash and cash equivalents at beginning of period	209,298	22,357
Cash and cash equivalents at end of period	263,237	203,397
Supplemental schedule of cash flows information:
Cash paid for interest	2
Supplemental schedule of non-cash investing and financing activities:
Conversion of convertible notes and accrued interest into equity		50,501
Discount from conversion of securities into equity		5,612
Conversion of convertible securities into equity		2,525
Employee stock purchase plan shares issued	361
Accrued offering costs		$ 1,554
Tenant improvement allowance receivable	2,633
Issuance of stock to purchase T-Cell Factory, B.V.	$ 4,209