Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2013
Income Tax Disclosure [Abstract]  
Schedule of Deferred Tax Assets
The components of the net deferred tax asset with the approximate income tax effect of each type of carryforward, credit and temporary differences are as follows.
(Dollars in thousands)
December 31, 2012
 
December 31, 2013
Deferred tax assets
 
 
 
   Operating loss carryforwards
$
31,800

 
$
41,141

   Tax credit carryforwards
2,203

 
5,529

   Other temporary differences
1,816

 
2,327

Tax assets before valuation allowance
35,819

 
48,997

Less: valuation allowance
(35,819
)
 
(48,997
)
Net deferred taxes
$
—

 
$
—

 
 
 
 
Schedule of Effective Tax Rate
The effective tax rate differs from the statutory tax rate due to the following:
 
Year ended December 31,
 
2011
 
2012
 
2013
U.S. Federal statutory rate
35.0
 %
 
35.0
 %
 
35.0
 %
State taxes
5.1

 
5.0

 
5.0

Research credits
3.4

 
—

 
13.3

Stock-based compensation
(0.4
)
 
(0.5
)
 
(0.5
)
Other
(0.1
)
 
(0.2
)
 
(0.1
)
Valuation allowance
(43.0
)
 
(39.3
)
 
(52.7
)
 
—
 %
 
—
 %
 
—
 %