Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Common shares of CO2 Solutions \| Fair Value, Inputs, Level 2
Investment [Line Items]
Fair value of common shares (in shares)	10,000,000
Available-for-sale Securities [Member]
Investment [Line Items]
Available-for-sale Securities	$ 588	$ 671
Available-for-sale Securities [Member] \| Fair Value, Inputs, Level 2
Investment [Line Items]
Available-for-sale Securities	$ 0	$ 671

Balance held at close of period in number of shares.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant