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Income Taxes (Tax Rates) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statutory federal tax rate (34.00%) (34.00%) (34.00%)
State income taxes, net of federal benefit (3.00%) (3.00%) (3.00%)
Research and development and other tax credits (2.00%) (2.00%) (3.00%)
Release of FIN 48 liability 0.00% 0.00% (2.00%)
Other (1.00%) 2.00% 1.00%
Change in tax rate 70.00% 0.00% 0.00%
Change in valuation allowance (30.00%) 37.00% 41.00%
Income tax provision 0.00% 0.00% 0.00%