Fair Value Measurements - Financial Assets measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	$ 7,449	$ 12,314
Total	7,449	12,314
Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	6,778	11,172
Common shares of CO2 Solutions	6,778	11,172
Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total	6,778	11,172
Level 1 | Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	6,778	11,172
Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total	671	1,142
Level 2 | Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	0	0
Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total	0	0
Level 3 | Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	0	0
Common shares of CO2 Solutions
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	671	1,142
Common shares of CO2 Solutions | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	0	0
Common shares of CO2 Solutions | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	671	1,142
Common shares of CO2 Solutions | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	$ 0	$ 0