Segment Reporting - Reconciliation of Segment Earnings to Earnings Before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Operating earnings	$ (643)	$ 4,298	$ 4,610	$ 5,375
Gain on dissolution of investment	75	0	573	0
Interest income	731	498	1,387	885
Interest expense	(117)	(15)	(182)	(34)
Earnings before income taxes	46	4,781	6,388	6,226
Reporting Segments
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Operating earnings	1,939	6,582	9,876	9,942
General and administrative
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Operating earnings	(2,582)	(2,284)	(5,266)	(4,567)
Segment reconciling items
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Gain on dissolution of investment	75	0	573	0
Interest income	731	498	1,387	885
Interest expense	$ (117)	$ (15)	$ (182)	$ (34)

Amount of gain (loss) from deconsolidation of subsidiary and derecognition of group of assets constituting transfer of business or nonprofit activity, excluding conveyance of oil and gas mineral rights and transfer of good or service in contract with customer.

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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The net result for the period of deducting operating expenses from operating revenues.

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