CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating activities
Net loss	$ (32,822,796)	$ (31,475,655)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	679,675	666,174
Stock-based compensation	7,889,999	5,221,217
Loss on disposal of property and equipment		7,487
Accretion of bond premium	331,881	1,027,922
Change in operating assets and liabilities:
Receivables	170,934	827,022
Prepaid expenses and other assets	(490,175)	(448,548)
Accounts payable	104,079	(421,206)
Accrued wages, benefits and other liabilities	(1,933,414)	(1,233,116)
Deferred revenue	(37,500)	(37,500)
Net cash used in operating activities	(26,107,317)	(25,866,203)
Investing activities
Purchases of property and equipment	(664,673)	(255,623)
Purchases of investments	(114,900,851)	(51,747,015)
Proceeds from sale and maturities of investments	160,865,053	62,899,199
Net cash provided by investing activities	45,299,529	10,896,561
Financing activities
Stock repurchase	(158,283)	(57,273)
Proceeds from the exercise of stock options	129,306	339,846
Proceeds from stock purchases under employee stock purchase plan	137,925	169,952
Net cash provided by financing activities	108,948	452,525
Effect of exchange rate		(1,158)
Net increase (decrease) in cash and cash equivalents	19,301,160	(14,518,275)
Cash and cash equivalents at beginning of period	15,431,622	45,533,443
Cash and cash equivalents at end of period	$ 34,732,782	$ 31,015,168