Cash Equivalents and Marketable Securities (Components of Cash Equivalents and Marketable Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 11,735	$ 11,683
Gross Unrealized Gains	579	986
Gross Unrealized Losses	0	0
Estimated Fair Value	12,314	12,669
Common shares of CO2 Solutions
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	563	563
Gross Unrealized Gains	579	986
Gross Unrealized Losses	0	0
Estimated Fair Value	1,142	1,549
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	11,172	11,120
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 11,172	$ 11,120