Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	1,150,961	238,153	120,341
Stock-based compensation expense	6,769,804	4,582,679	2,450,579
Other non-cash charges	36,607	41,103	16,500
Amortization of premium (discount) on marketable	728,552	871,215	366,231
Loss on disposal of fixed assets	2,283,344	149,983
Premium paid on securities purchased	(542,567)
Changes in operating assets and liabilities:
Accounts receivable	95,285
Prepaid expenses, other current and long-term assets	(2,872,955)	(526,247)	(320,352)
Accounts payable	(992,739)	1,581,164	518,032
Accrued expenses and other current and long-term liabilities	2,765,899	428,769	1,017,412
Net cash used in operating activities	(62,471,768)	(38,948,632)	(23,144,921)
Cash flows from investing activities
Purchases of property and equipment	(8,439,577)	(5,197,326)	(802,481)
Change in restricted cash	(400,000)	48,000
Purchases of marketable securities	(196,060,561)	(145,797,556)	(79,383,553)
Sale and redemption of marketable securities	72,147,035	137,702,156	30,735,000
Net cash used in investing activities	(132,753,103)	(13,244,726)	(49,451,034)
Cash flows from financing activities
Payment of debt issuance costs	(42,034)	(107,741)
Payments on debt		(3,500,000)	(1,500,000)
Proceeds from borrowings under term loan	15,000,000	15,003,533
Payments of public offering costs	(293,368)	(224,648)	(1,517,484)
Proceeds from the issuance of common stock	147,889,139		162,137,580
Proceeds from the exercise of stock options	166,816	592,787	304,496
Proceeds from Employee Stock Purchase Plan	475,648	275,949	80,631
Net cash provided by financing activities	163,196,201	12,039,880	159,505,223
Net increase (decrease) in cash and cash equivalents	(32,028,670)	(40,153,478)	86,909,268
Cash and cash equivalents at beginning of period	62,944,044	103,097,522	16,188,254
Cash and cash equivalents at end of period	30,915,374	62,944,044	103,097,522
Supplemental disclosures of cash flow information:
Cash paid for interest	1,296,875	572,340	364,889
Supplemental disclosures of non-cash financing activities:
Public offering costs included in accounts payable or accrued	94,606		224,648
Conversion of convertible preferred stock into common stock			74,806,213
Purchases of property and equipment in accounts payable and accrued expenses	$ 622,005	$ 1,575,907	$ 52,011