Cash Equivalents and Marketable Securities - Components of Cash Equivalents and Marketable Securities (Details) $ in Thousands	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) security
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	$ 7,364	$ 7,341
Gross Unrealized Gains	134	108
Gross Unrealized Losses	0	0
Total	7,498	$ 7,449
Number of marketable securities in unrealized loss position | security		0
Common shares of CO2 Solution [Member]
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	563	$ 563
Gross Unrealized Gains	134	108
Gross Unrealized Losses	0	0
Total	697	671
Money Market Funds [Member]
Cash Equivalents and Marketable Securities [Line Items]
Adjusted Cost	6,801	6,778
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total	$ 6,801	$ 6,778