Investments in Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended
	Nov. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair value of financial assets measured at fair value on a recurring basis				$ 180,881		$ 183,173
Noncash Dividend Income				4,883	$ 3,926
Purchases of preferred stock and warrants				0	$ 1,161
Preferred stock
Schedule of Investments [Line Items]
Fair value of financial assets measured at fair value on a recurring basis				$ 166,069		161,225
Fibrocell Science, Inc.
Schedule of Investments [Line Items]
Convertible preferred shares issued (in shares)		1,161			1,161
Number of common shares into which warrants are convertible (in shares)		498,843
Convertible preferred shares, dividend rate				4.00%
ZIOPHARM Oncology, Inc.
Schedule of Investments [Line Items]
Preferred shares, stated value (in usd per share)			$ 1,200
Threshold consecutive trading days, conversion of shares				20 days
Preferred shares, dividend rate (in usd per share)				$ 12.00
Preferred shares, dividend rate, for unconverted shares after conversion event (in usd per share)				24.00
ZIOPHARM Oncology, Inc. | Minimum
Schedule of Investments [Line Items]
Preferred shares, conversion calculation, divisor (in usd per share)				$ 1.00
Other Contractual Payments | Upfront and Milestone Payments | ZIOPHARM Oncology, Inc.
Schedule of Investments [Line Items]
Collaborative arrangement consideration received, number of preferred shares (in shares)			100,000
Oragenics, Inc. | Preferred stock
Schedule of Investments [Line Items]
Fair value of financial assets measured at fair value on a recurring basis				$ 0		0
Receivables converted to preferred stock	$ 3,385
Preferred shares, initial dividend rate				12.00%
Preferred shares, subsequent dividend rate				20.00%
ZIOPHARM Oncology, Inc.
Schedule of Investments [Line Items]
Dividend income, number of preferred shares received (in shares)				3,624	3,216
Noncash Dividend Income				$ 4,871	$ 3,923
ZIOPHARM Oncology, Inc. | Preferred stock
Schedule of Investments [Line Items]
Fair value of financial assets measured at fair value on a recurring basis				165,703		160,832
Fibrocell Science, Inc.
Schedule of Investments [Line Items]
Purchases of preferred stock and warrants		$ 1,161
Fibrocell Science, Inc. | Preferred stock
Schedule of Investments [Line Items]
Fair value of financial assets measured at fair value on a recurring basis				$ 366		$ 393