Cash Equivalents and Marketable Securities - Components of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	$ 31,476
Gross Unrealized Gains	113
Gross Unrealized Losses	0
Estimated Fair Value	31,589
Adjusted Cost		$ 7,341
Gross Unrealized Gains		108
Gross Unrealized Losses		0
Estimated Fair Value		7,449
Common shares of CO2 Solution [Member]
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	563
Gross Unrealized Gains	113
Gross Unrealized Losses	0
Estimated Fair Value	676
Adjusted Cost		563
Gross Unrealized Gains		108
Gross Unrealized Losses		0
Estimated Fair Value		671
Money Market Funds [Member]
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	30,913
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	30,913
Adjusted Cost		6,778
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value	$ 30,900	$ 6,778