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Income Taxes - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Tax Disclosure [Line Items]      
Increase in valuation allowance $ 46,536,000 $ 31,170,000  
Income tax expense (118,085,000) (75,777,000) $ (33,947,000)
Deduction for option exercises $ 4,600,000 3,824,000  
Period over which certain cumulative changes in the ownership interest of significant shareholders will be considered, subject to an annual limitation of net operating loss and tax credit carryforwards 3 years    
Percentage of cumulative changes in the ownership interest of significant shareholders that will be considered, subject to an annual limitation of net operating loss and tax credit carryforwards 50.00%    
Accrued interest or penalties related to uncertain tax positions $ 0 0  
Interest or penalties related to uncertain tax positions recognized in statements of operations and comprehensive loss 0 0  
Uncertain tax position 0 0  
Germany [Member]      
Income Tax Disclosure [Line Items]      
Income tax expense 96,000    
Internal Revenue Service (IRS) [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards 166,680,000 105,628,000  
Internal Revenue Service (IRS) [Member] | Research Tax Credit Carryforward [Member]      
Income Tax Disclosure [Line Items]      
Tax credit carryforwards 19,988,000 3,042,000  
State and Local Jurisdiction [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards 164,572,000 104,259,000  
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]      
Income Tax Disclosure [Line Items]      
Tax credit carryforwards $ 2,428,000 $ 1,695,000