Fair Value Measurements - Financial Assets measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	$ 12,314	$ 12,669
Total	12,314	12,669
Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	11,172	11,120
Total financial assets measured at fair value on a recurring basis	11,172	11,120
Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total	11,172	11,120
Level 1 | Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Money market funds	11,172	11,120
Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total	1,142	1,549
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
Total financial assets measured at fair value on a recurring basis	$ 1,142	1,549
Number of CO2 Solutions common shares we invested in (shares)	10,000,000
Common shares of CO2 Solutions | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	$ 0	0
Common shares of CO2 Solutions | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	1,142	1,549
Common shares of CO2 Solutions | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	$ 0	$ 0