CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net loss	$ (302,139)	$ (245,874)	$ (145,618)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	68,014	48,309	24,884
Amortization of premium on investment securities, net	1,706	4,842	5,637
Depreciation and amortization	5,825	3,424	1,384
Foreign currency remeasurement (gain) loss	(7,018)	1,322
Non-cash license fee from collaboration arrangement		700
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,338)	(7,147)	(7,131)
Other assets	522	(1,242)	179
Accounts payable	3,459	2,502	(1,982)
Accrued liabilities and other liabilities	(4,628)	32,189	16,670
Deferred tax liabilities	(16,246)
Net cash used in operating activities	(253,843)	(160,975)	(105,977)
Investing activities:
Acquisition, net of cash acquired	(142,804)
Purchase of property and equipment	(2,793)	(10,188)	(4,955)
Purchase of investments	(230,487)	(442,490)	(624,226)
Proceeds from sale of investments	157,934	140,556	89,321
Proceeds from maturities of investments	273,632	403,239	249,050
Decrease (increase) in restricted cash	934	(1,202)	(1,541)
Net cash provided by (used in) investing activities	56,416	89,915	(292,351)
Financing activities:
Proceeds from the issuance of common stock in connection with underwritten public offerings, net			461,136
Proceeds from the issuance of common stock in connection with at-the-market offering, net	131,960	79,486
Proceeds from the issuance of common stock in connection with collaboration agreement, net		51,356
Proceeds from the issuance of common stock under equity plan awards, net of tax	9,251	7,834	6,437
Repayment of note payable	(4,944)
Net cash provided by financing activities	136,267	138,676	467,573
Effect of exchange rate changes on cash	528	(65)
Net increase (decrease) in cash and cash equivalents	(60,632)	67,551	69,245
Cash and cash equivalents at beginning of year	161,120	93,569	24,324
Cash and cash equivalents at end of year	100,488	161,120	93,569
Supplemental disclosures of non-cash investing and financing information:
Fair value of vested stock options assumed in acquisition	8,979
Costs of property and equipment included in accounts payable and accrued liabilities	$ 400	147	$ 769
Tenant improvement allowance		$ 3,467