Cash Equivalents and Equity Securities - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2009	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Equity Securities, Available-for-sale [Line Items]
Cash and cash equivalents		$ 53,039	$ 31,219	$ 19,240
Money market funds		31,200	6,800
Cash		21,800	24,400
Initial ownership percentage at the time of investment	16.60%
Statutory resale restriction expiry period	4 months
Gross unrealized gains			108
Cumulative effect of change in accounting principles		(4,059)
Unrealized loss on investments		(83)	$ 0	$ 0
Accounting Standards Update 2016-01
Equity Securities, Available-for-sale [Line Items]
Cumulative effect of change in accounting principles					$ 500
Common shares of CO2 Solutions
Equity Securities, Available-for-sale [Line Items]
Number of CO2 Solutions common shares we invested in (shares)	10,000,000
Unrealized loss on investments		$ (84)