Commitments and Contingencies - Additional Information (Detail)											1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 13, 2015 USD ($)	Sep. 28, 2015 USD ($)	May. 22, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jan. 13, 2015 USD ($) shares	Jan. 09, 2015 USD ($) shares	Aug. 30, 2013 ft²	Mar. 07, 2011 USD ($)	Jan. 06, 2011	Aug. 24, 2004 Patent	Jul. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) ft² $ / shares shares	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($) shares	Sep. 30, 2015 USD ($) ft² Agreement $ / shares shares	Sep. 30, 2014 USD ($) shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($) ft²	Dec. 31, 2011 USD ($) ft²	May. 31, 2015 USD ($)	Oct. 15, 2014 USD ($)	Nov. 02, 2012 USD ($)	Jun. 30, 2012 USD ($) ft²	Jan. 31, 2012 ft²
Commitments and Contingencies Disclosure [Line Items]
Loss on disposal of fixed assets															$ 1,000
Total rent Expense															581,000	$ 445,000
Deferred rent - current portion												$ 402,000			402,000		$ 280,000
Deferred rent-non-current portion												379,000			379,000		570,000
Deferred rent liability net												781,000			781,000		850,000
Research and Development Expense												16,970,000		$ 9,733,000	98,643,000	24,621,000
Research and development arrangement Terms					Pursuant to the Research and Development Agreement, the Company, Intrexon and MD Anderson have agreed to form a joint steering committee that will oversee and manage the new and ongoing research programs. As provided under the MD Anderson License, the Company will provide funding for research and development activities in support of the research programs under the Research and Development Agreement for a period of three years and in an amount of no less than $15 million and no greater than $20 million per year.
Research and Development Service Agreement Quarterly Payment															7,500,000
Offset costs in research and development expense															372,000
Deferred revenue - current portion												6,933,000			6,933,000		1,360,000
Deferred Revenue, long term												49,514,000			49,514,000
Research contract revenue												1,869,000		633,000	2,413,000	1,033,000
Prepaid Expenses and Other Current Assets
Commitments and Contingencies Disclosure [Line Items]
Pre paid research and development expenses												$ 7,100,000			$ 7,100,000
Subsequent Event
Commitments and Contingencies Disclosure [Line Items]
Research and Development Service Agreement Quarterly Payment	$ 3,800,000
DEKK Tec Inc
Commitments and Contingencies Disclosure [Line Items]
Milestone maximum payment																						$ 4,000,000
Option to purchase common stock | shares															27,616
Sale of stock price per share | $ / shares												$ 0.02			$ 0.02
Outstanding options to purchase common stock | shares												13,808			13,808
Southern Research Institute
Commitments and Contingencies Disclosure [Line Items]
Milestone maximum payment																	775,000
Royalty payment																	25,000	$ 25,000	$ 25,000
The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Commitments and Contingencies Disclosure [Line Items]
Milestone maximum payment														$ 4,500,000		$ 4,500,000
Option to purchase common stock | shares															50,222
Number of products | Patent										2
The University of Texas MD Anderson Cancer Center and The Texas A & M University System | Upon enrollment of the first patient in a multi-center pivotal clinical trial
Commitments and Contingencies Disclosure [Line Items]
Shares expected to vest | shares														12,556		12,556
Solasia
Commitments and Contingencies Disclosure [Line Items]
Upfront payment received								$ 5,000,000
Deferred revenue - current portion												$ 544,000			$ 544,000
Research contract revenue												272,000		$ 633,000	816,000	$ 1,000,000
Estimated upfront payment for research and development funding earning period																75 months
Solasia | Development-based milestones
Commitments and Contingencies Disclosure [Line Items]
Expected Additional milestone payments to be received															32,500,000
Solasia | Sales-based milestones
Commitments and Contingencies Disclosure [Line Items]
Expected Additional milestone payments to be received															53,500,000
Baxter Healthcare Corporation | Upon the successful U.S. IND application for indibulin
Commitments and Contingencies Disclosure [Line Items]
Installment payments															$ 0		250,000	250,000
Novella Clinical, Inc.
Commitments and Contingencies Disclosure [Line Items]
Milestone maximum payment																							$ 790,000
Number of amendments signed | Agreement															2
Novella Clinical, Inc. | Upon three clinical milestones
Commitments and Contingencies Disclosure [Line Items]
Installment payments													$ 10,000				236,000
Minimum
Commitments and Contingencies Disclosure [Line Items]
Research and Development Expense					$ 15,000,000
Maximum
Commitments and Contingencies Disclosure [Line Items]
Research and Development Expense					20,000,000
License Agreement
Commitments and Contingencies Disclosure [Line Items]
Research and Development Expense															$ 67,300,000
Issuance of common stock in licensing agreement, shares | shares															11,722,163
Merck License
Commitments and Contingencies Disclosure [Line Items]
Agreement termination, notice period															90 days
Payments for development and commercial milestones per Product															$ 413,000,000
Milestone payments, percentage															50.00%
Agreement commencement date															2015-05
Deferred revenue, revenue recognized												1,600,000			$ 1,600,000
Deferred revenue, upfront payment												55,900,000			$ 55,900,000						$ 57,500,000
Intrexon Corporation
Commitments and Contingencies Disclosure [Line Items]
Contract termination description															The Company's obligation to pay 50% of net profits or revenue described above with respect to these "retained" products will survive termination of the Channel Agreement.
Licensing fee		$ 10,000,000									$ 115,000,000
Milestone payment receivable															$ 5,000,000
Milestone payment receivable period															2 years
Upfront payment received											$ 57,500,000
Percentage of upfront fee Payable											50.00%
Milestone maximum payment												50,000,000			$ 50,000,000
Intrexon Corporation | Quarterly Payment
Commitments and Contingencies Disclosure [Line Items]
Percent of company net profit									50.00%
Percentage of revenue agreed to pay which is obtained from sublicensor									50.00%
Intrexon Corporation | Letter Agreement
Commitments and Contingencies Disclosure [Line Items]
Cash consideration for license agreement						$ 7,500,000
Intrexon Corporation | License Agreement
Commitments and Contingencies Disclosure [Line Items]
Cash consideration for license agreement					$ 50,000,000
Intrexon Corporation | GVHDA Arrangement
Commitments and Contingencies Disclosure [Line Items]
Percent of company net profit															50.00%
Contract termination description															The Company's obligation to pay 50% of net profits or revenue with respect to these "retained" products will survive termination of the GvHD Agreement.
Licensing fee		$ 10,000,000
Agreement termination period		24 months
Agreement termination, notice period		90 days
Research and Development Expense															$ 10,000,000
Ziopharm | Letter Agreement
Commitments and Contingencies Disclosure [Line Items]
Common stock issued for cash | shares						1,597,602
Cash consideration for license agreement						$ 7,500,000
Ziopharm | License Agreement
Commitments and Contingencies Disclosure [Line Items]
Common stock issued for cash | shares					10,124,561
Cash consideration for license agreement					$ 50,000,000
New York, NY
Commitments and Contingencies Disclosure [Line Items]
Office space under operating lease | ft²																			7,259	6,251
Letter of credit												$ 388,000			$ 388,000				$ 388,000	$ 388,000
Additional office space under operating lease | ft²																									1,008
Operating lease expiration month and year																			2018-10
Loss on sublease																		729,000
Remaining contractual obligation																		2,300,000
Sublease revenue from subtenant																		1,600,000
Loss on disposal of fixed assets																		(392,000)
Boston, MA
Commitments and Contingencies Disclosure [Line Items]
Office space under operating lease | ft²												21,184			21,184
Operating lease expiration month and year			2016-08																	2016-08
Loss on sublease																		42,000
Remaining contractual obligation																		367,000
Sublease revenue from subtenant			$ 105,000															$ 325,000
Security deposits			$ 17,000									$ 127,000			$ 127,000
Area under sublease agreement | ft²							5,249
Security deposit from subtenant																	$ 20,000
Boston, MA | The first floor
Commitments and Contingencies Disclosure [Line Items]
Loss on sublease				$ 167,000
Operating lease space | ft²																				5,249
Boston, MA | The second floor
Commitments and Contingencies Disclosure [Line Items]
Office space under operating lease | ft²																								4,113
Operating lease space | ft²																				8,538
Boston, MA | The third floor
Commitments and Contingencies Disclosure [Line Items]
Operating lease space | ft²																				6,959
Boston, MA | The four floor
Commitments and Contingencies Disclosure [Line Items]
Office space under operating lease | ft²																								9,800
Security deposits																								$ 41,000