Summary of Significant Accounting Policies - Summarized Financial Data of Equity Method Investments (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investment, Summarized Financial Information, Gross Profit (Loss) [Abstract]
Operating expenses										$ 368,871	$ 316,092	$ 320,469
Operating loss	$ (26,492)	[1]	$ (44,747)	$ (35,270)	$ (31,381)	$ (32,590)	$ (28,821)	$ (23,222)	$ (40,533)	(137,890)	(125,166)	(146,864)
Other, net										22,473	(47,865)	68,830
Equity Method Investments
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Current assets	61,086					77,761				61,086	77,761
Non-current assets	13,598					11,040				13,598	11,040
Total assets	74,684					88,801				74,684	88,801
Equity Method Investment, Summarized Financial Information, Liabilities [Abstract]
Current liabilities	6,213					11,588				6,213	11,588
Net assets	$ 68,471					$ 77,213				68,471	77,213
Equity Method Investment, Summarized Financial Information, Gross Profit (Loss) [Abstract]
Revenues, net										254	417	1,176
Operating expenses										41,904	62,373	32,513
Operating loss										(41,650)	(61,956)	(31,337)
Other, net										(8)	1,535	(64)
Net loss										$ (41,658)	$ (60,421)	$ (31,401)

[1] During the fourth quarter of 2017, the Company recognized the remaining balance of deferred revenue associated with ZIOPHARM ECC2 upon the parties' mutual agreement to terminate (Note 5). The Company also recorded goodwill impairment charges primarily related to the AquaBounty reporting unit and an impairment charge related to certain of its in-process research and development assets (Note 11).