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Significant Accounting Policies - Cash, Cash Equivalents, and Short-Term Investments (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Significant Accounting Policies    
Cash equivalents $ 12.5 $ 1.5
Cash and cash equivalents maintained outside the United States (as a percent) 77.00% 54.00%