Stockholders' Equity (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2007 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Oct. 31, 2011 USD ($)
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | shares		30,000,000	30,000,000
Number of demand rights to require the Company to conduct registered underwritten public offering		3
Maximum shares the collaborator could sell | shares		1,400,000
Repayments of Convertible Debt		$ 0	$ 12,894,000	$ 166,467,000
Gain (Loss) on Extinguishment of Debt				18,861,000
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			47,789,000	819,657,000
Payments for Repurchase of Warrants		$ 0	$ 643,365,000	573,487,000
Maximum amount of warrant payable				$ (23,317,000)
Convertible senior notes
Class of Stock [Line Items]
Total convertible senior notes - par					$ 400,000,000.0
Sanofi
Class of Stock [Line Items]
Number of shares of Common Stock purchased | shares	12,000,000
Maximum percentage of outstanding shares that may be acquired, under 'standstill' provisions		30.00%
Percentage Of Outstanding Shares Or Voting Rights Under Amended Investor Agreement		20.00%
Percentage Of Outstanding Shares that the Company is required to appoint an individual agreed upon by the Company and the Collaborator to the Company's board of directors		20.00%
Class A Stock
Class of Stock [Line Items]
Common Stock, shares authorized (in shares) | shares		40,000,000	40,000,000
Common Stock, par value (in dollars per share) | $ / shares		$ 0.001	$ 0.001
Common Stock, voting rights		ten votes per share
Common Stock
Class of Stock [Line Items]
Common Stock, shares authorized (in shares) | shares		320,000,000	320,000,000
Common Stock, par value (in dollars per share) | $ / shares		$ 0.001	$ 0.001
Common Stock, voting rights		one vote per share
Debt Conversion, Converted Instrument, Shares Issued | shares			121,000	1,625,000
Bayer Collaboration Agreement - PDGFR-beta outside the US
Class of Stock [Line Items]
Maximum percentage of outstanding shares that may be acquired, under 'standstill' provisions		20.00%
Teva Pharmaceuticals [Member]
Class of Stock [Line Items]
Maximum percentage of outstanding shares that may be acquired, under 'standstill' provisions		5.00%
Additional Paid-in Capital
Class of Stock [Line Items]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			$ 47,789,000	$ 819,657,000
Maximum amount of warrant payable				$ (23,317,000)