Stockholders' Equity (Details) (USD $)	1 Months Ended		12 Months Ended			1 Months Ended
	Nov. 30, 2014	Oct. 31, 2011	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2007	Jan. 31, 2014
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			30,000,000	30,000,000
Number of demand rights to require the Company to conduct registered underwritten public offering			3
Debt Conversion			$ 230,600,000
Total convertible senior notes - par				400,000,000
Equity component of convertible senior notes, net of issuance costs		120,600,000
Proceeds received from the Warrant		93,800,000
Repayments of Convertible Debt			220,639,000	0	0
Gains (Losses) on Extinguishment of Debt			(33,469,000)	0	0
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			691,869,000
Payments for Repurchase of Warrants			294,552,000	0	0
Maximum reduction of warrants held by warrant holder	493,229
Maximum amount of warrant payable			(148,496,000)
Maximum number of shares issuable upon exercise of warrants			3,540,095	4,760,840
Call option
Class of Stock [Line Items]
Purchase of convertible note hedges		117,500,000
Warrant Transactions
Class of Stock [Line Items]
Strike price of the warrant transaction (in dollars per share)		103.41
Proceeds received from the Warrant		93,800,000
Convertible senior notes
Class of Stock [Line Items]
Total convertible senior notes - par		400,000,000.0	169,400,000	400,000,000
Sanofi
Class of Stock [Line Items]
Number of shares of Common Stock purchased						12,000,000
Cash price of newly issued, unregistered shares of Common Stock purchased						312,000,000
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)			40,000,000	40,000,000
Common Stock, par value (in dollars per share)			$ 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)			160,000,000	160,000,000
Common Stock, par value (in dollars per share)			$ 0.001	$ 0.001
Common Stock, voting rights			one vote per share
Debt Conversion, Converted Instrument, Shares Issued			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%				20.00%
Additional Paid-in Capital
Class of Stock [Line Items]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			691,869,000
Maximum amount of warrant payable			$ (148,496,000)