Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receipt of preferred stock as consideration for amendments to collaboration agreements	$ 0	$ 120,000	$ 0
Purchase of preferred stock	1,161	2,308	17,080
Conversion of receivables to preferred stock	3,385	0	0
Dividend income from investments in preferred stock	16,756	7,421	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	129,545	0
Receipt of preferred stock as consideration for amendments to collaboration agreements	0	120,000
Purchase of preferred stock	766	0
Conversion of receivables to preferred stock	3,385	0
Dividend income from investments in preferred stock	16,756	7,421
Net unrealized appreciation in the fair value of the investments in preferred stock	10,773	2,124
Ending balance	$ 161,225	$ 129,545	$ 0