CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (490,874)	$ (410,108)	$ (290,073)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	13,367	15,130	19,050
Stock-based compensation	92,819	75,528	45,783
Charge for 401(k) company stock match	2,302	1,507	984
Realized loss (gain) on sale of marketable equity securities	1,894	(6,977)
Other	608
Changes in operating assets and liabilities:
Proceeds from landlord tenant improvements		2,145	374
Billed and unbilled collaboration receivables	(10,668)	(15,036)	31,639
Prepaid expenses and other assets	(20,711)	(5,276)	(6,820)
Accounts payable	(4,939)	10,098	1,676
Accrued expenses and other	31,568	10,673	6,343
Deferred revenue	1,848	14,615	1,904
Net cash used in operating activities	(382,786)	(307,701)	(189,140)
Cash flows from investing activities:
Purchases of property, plant and equipment	(104,209)	(64,557)	(12,950)
Purchases of restricted investments and increase in restricted cash		(150,000)	(1,471)
Purchases of marketable securities	(903,457)	(759,310)	(1,033,843)
Sales and maturities of marketable securities	597,305	1,116,458	726,943
Sale of restricted investments	120,000
Net cash (used in) provided by investing activities	(290,361)	142,591	(321,321)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	84,142	14,127	31,137
Proceeds from issuance of common stock, net of offering costs	1,139,625		496,400
Proceeds from issuance of common stock to Sanofi Genzyme	21,381	14,301	89,018
Proceeds from issuance of long-term debt		150,000
Repayment of long-term debt	(120,000)
Payments for repurchase of common stock for employee tax withholding	(257)	(596)	(378)
Net cash provided by financing activities	1,124,891	177,832	616,177
Net increase in cash and cash equivalents	451,744	12,722	105,716
Cash and cash equivalents, beginning of period	193,617	180,895	75,179
Cash and cash equivalents, end of period	645,361	193,617	180,895
Supplemental disclosure of cash flows:
Cash paid for interest	2,430	1,096
Cash paid for income taxes	114	111	66
Supplemental disclosure of noncash investing and financing activities:
Fixed asset expenditures included in accounts payable and accrued expenses	8,176	33,153	1,333
Fair value of common stock received for collaboration agreement in other assets	2,700
Receipt of common stock for exercises of stock options	653	$ 1,260	$ 686
Repurchase of common stock for employee tax withholding in accrued expenses	$ 450