Prior-Period Financial Statements - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 35,292	$ 16,528
Marketable securities	220,116	207,603
Accounts receivable	1,283	1,142
Prepaid expenses and other current assets	12,074	5,606
Total current assets	269,527	230,879
Property and equipment, net	9,401	7,088
Patents, licenses, and other intangible assets, net	11,531	11,148
Marketable securities - long term	299,971	139,198
Income tax receivable	762	1,524
Loan receivable		86
Interest receivable		14
Other assets	265	265
Total assets	591,457	390,202
Current liabilities
Accounts payable	4,916	6,869
Accrued expenses	5,923	5,480
Current portion of deferred rent	199	26
Current portion of deferred revenue	60,118	60,118
Income taxes		157
Total current liabilities	71,156	72,650
Deferred rent, less current portion	1,122	1,088
Total liabilities	72,278	73,738
Commitments and contingencies
Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at December 31, 2017
Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017	558	470
Additional paid-in capital	828,858	570,670
Accumulated other comprehensive loss	(2,008)	(1,808)
Accumulated deficit	(308,229)	(252,868)
Stockholders' equity	519,179	316,464
Total liabilities and stockholders' equity	$ 591,457	390,202
As Reported Before ASC Topic 606
Current liabilities
Stockholders' equity		306,025	$ 337,933
ASU 2014-09 | As Reported Before ASC Topic 606
Current assets
Cash and cash equivalents		16,528
Marketable securities		207,603
Accounts receivable		1,142
Prepaid expenses and other current assets		5,606
Total current assets		230,879
Property and equipment, net		7,088
Patents, licenses, and other intangible assets, net		11,148
Marketable securities - long term		139,198
Income tax receivable		1,524
Other assets		265
Total assets		390,102
Current liabilities
Accounts payable		6,869
Accrued expenses		5,480
Current portion of deferred rent		26
Current portion of deferred revenue		88,813
Income taxes		157
Total current liabilities		101,345
Deferred rent, less current portion		1,088
Deferred revenue, less current portion		5,623
Total liabilities		108,056
Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017		470
Additional paid-in capital		570,670
Accumulated other comprehensive loss		(1,808)
Accumulated deficit		(287,286)
Stockholders' equity		282,046
Total liabilities and stockholders' equity		390,102
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Current assets
Loan receivable		86
Interest receivable		14
Total assets		100
Current liabilities
Current portion of deferred revenue		(28,695)
Total current liabilities		(28,695)
Deferred revenue, less current portion		(5,623)
Total liabilities		(34,318)
Accumulated deficit		34,418
Stockholders' equity		34,418
Total liabilities and stockholders' equity		$ 100