Cash Equivalents and Marketable Securities - Components of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 7,341	$ 11,735
Gross Unrealized Gains	108	579
Gross Unrealized Losses	0	0
Estimated Fair Value	7,449	12,314
Common shares of CO2 Solutions
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	563	563
Gross Unrealized Gains	108	579
Gross Unrealized Losses	0	0
Estimated Fair Value	671	1,142
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	6,778	11,172
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 6,778	$ 11,172