Fair Value Measurements - Schedule of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	$ 419	$ 588
Total	73,586	31,813
Available-for-sale Securities [Member]
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	419	588
Money Market Funds [Member]
Summary of financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	73,167	31,225
Level 1 [Member]
Summary of financial instruments measured at fair value on a recurring basis
Total	73,586	31,813
Level 1 [Member] | Available-for-sale Securities [Member]
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	419	588
Level 1 [Member] | Money Market Funds [Member]
Summary of financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	73,167	31,225
Level 2 [Member]
Summary of financial instruments measured at fair value on a recurring basis
Total	0	0
Level 2 [Member] | Available-for-sale Securities [Member]
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	$ 0	$ 0
Level 2 [Member] | Common Shares of Co Two Solution [Member]
Summary of financial instruments measured at fair value on a recurring basis
Investment owned (in shares)	10,000,000	10,000,000
Level 2 [Member] | Money Market Funds [Member]
Summary of financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	$ 0	$ 0
Level 3 [Member]
Summary of financial instruments measured at fair value on a recurring basis
Total	0	0
Level 3 [Member] | Available-for-sale Securities [Member]
Summary of financial instruments measured at fair value on a recurring basis
Common shares of CO2 Solutions	0	0
Level 3 [Member] | Money Market Funds [Member]
Summary of financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	$ 0	$ 0