Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 item	Dec. 31, 2013	Dec. 31, 2012
Income Taxes
Provision for income taxes	$ 0	$ 0	$ 0
Deferred tax assets:
Net operating loss carryforwards	386,000,000	480,000,000
Deferred revenues	2,000,000	5,000,000
Capitalized research and development expenditures	0	5,000,000
Research and development tax credit carryforwards	53,000,000	44,000,000
Other	18,000,000	28,000,000
Total deferred tax assets	459,000,000	562,000,000
Valuation allowance	(459,000,000)	(562,000,000)
Net deferred tax assets	0	0
Information related to valuation allowance
Increase (decrease) in valuation allowance	(103,800,000)	70,100,000	3,000,000
Federal net operating losses	253,200,000
Valuation Allowance	88,600,000
Deferred tax assets	9,200,000
The differences between the U.S. federal statutory income tax rate to the Company's effective tax rate
U.S. federal statutory income tax rate (as a percent)	35.00%	34.00%	34.00%
Federal and state research credits (as a percent)	12.66%	3.63%	(4.21%)
Non-deductible executive compensation (as a percent)	(0.16%)	(0.07%)	(13.24%)
Stock-based compensation (as a percent)	(1.11%)	0.28%	(1.36%)
Expiration of net operating loss (as a percent)	0.00%	0.00%	(1.81%)
Effect of Spin-Off Transaction (as a percent)	(203.20%)	0.00%	0.00%
Other (as a percent)	(4.04%)	(2.51%)	(2.09%)
Change in valuation allowance (as a percent)	160.85%	(35.33%)	(11.29%)
Effective tax rate (as a percent)	0.00%	0.00%	0.00%
Gross unrecognized tax benefits
Gross unrecognized tax benefits at the beginning of the period	57,420,000	52,500,000	46,900,000
Gross decrease for tax positions for prior years	(42,650,000)	(565,000)	0
Gross increase in tax positions for current year	689,000	5,485,000	5,600,000
Gross unrecognized tax benefits at the end of the period	15,459,000	57,420,000	52,500,000
Accrued interest or penalties due to our net operating losses	0	0
Number of ownership changes	2
Federal
Operating Loss and Tax Credit Carryforward
Net operating loss carryfowards	1,158,300,000
Federal | Research and Development
Operating Loss and Tax Credit Carryforward
Tax credit carryforward amount	45,200,000
State
Operating Loss and Tax Credit Carryforward
Net operating loss carryfowards	1,008,600,000
State | Research
Operating Loss and Tax Credit Carryforward
Tax credit carryforward amount	$ 32,100,000