Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Net operating loss carry forwards	$ 202,289	$ 154,865
Research and development credit carry forwards	16,659	13,192
Stock-based compensation	15,476	9,386
Accruals and other	18,927	5,528
Depreciation	218	185
Deferred revenue	4,009	420
Deferred tax assets, gross	257,578	183,576
Valuation allowance	$ (257,578)	$ (183,576)

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from deferred income.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from other reserves and accruals not separately disclosed.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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