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Cash Equivalents and Marketable Securities - Components of Cash Equivalents and Marketable Securities (Details)
$ in Thousands
Sep. 30, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Cash Equivalents and Marketable Securities [Line Items]    
Adjusted Cost $ 6,825 $ 11,735
Gross Unrealized Gains 600 579
Gross Unrealized Losses 0 0
Total $ 7,425 $ 12,314
Number of marketable securities in unrealized loss position | security 0 0
Common shares of CO2 Solution [Member]    
Cash Equivalents and Marketable Securities [Line Items]    
Adjusted Cost $ 563 $ 563
Gross Unrealized Gains 600 579
Gross Unrealized Losses 0 0
Total 1,163 1,142
Money Market Funds [Member]    
Cash Equivalents and Marketable Securities [Line Items]    
Adjusted Cost 6,262 11,172
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Total $ 6,262 $ 11,172