Stockholders' Equity (Details)	1 Months Ended		12 Months Ended
	Oct. 31, 2011 USD ($)	Dec. 31, 2007 shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares
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
Repayments of Convertible Debt			$ 12,894,000	$ 166,467,000	$ 220,639,000
Gains (Losses) 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	691,869,000
Payments for Repurchase of Warrants			$ 643,365,000	573,487,000	294,552,000
Maximum amount of warrant payable				$ (23,317,000)	$ 148,496,000
Call option
Class of Stock [Line Items]
Purchase of convertible note hedges	$ 117,500,000
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	160,000,000
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	2,018,000
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%
maximum percentage of voting power			30.00%
Maximum percentage of voting power that may be acquired by another party			10.00%
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	$ 691,869,000
Maximum amount of warrant payable				$ (23,317,000)	$ 148,496,000