Fair Value Disclosure (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Centennial asset impairment charge	$ 0	$ 982
Long-term debt fair value		56,700
Long-term Debt	10,571	$ 56,700
Revolving Credit Agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	10,400
Level 1 | Fair Value, Measurements, Recurring | Bellaire
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) on equity securities	$ (300)
Accounting Standards Update 2016-01
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative effect of new accounting principles on retained earnings			$ 0
Accounting Standards Update 2016-01 | Retained Earnings
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative effect of new accounting principles on retained earnings			$ 2,727