Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]

Summary of financial instruments measured at fair value on a recurring basis

The following tables present the financial instruments that were measured at fair value on a recurring basis at December 31, 2017 and 2016 by level within the fair value hierarchy (in thousands):

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Money market funds	$	6,778	$	—	$	—	$	6,778
Common shares of CO2 Solutions	—		671		—		671
Total	$	6,778	$	671	$	—	$	7,449

	December 31, 2016
	Level 1		Level 2		Level 3		Total
Money market funds	$	11,172	$	—	$	—	$	11,172
Common shares of CO2 Solutions	—		1,142		—		1,142
Total	$	11,172	$	1,142	$	—	$	12,314