Long-Term Debt (Narrative) (Details)		3 Months Ended		9 Months Ended
	Nov. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Covenant To Actual Ratios [Line Items]
Revolving credit facility, maximum borrowing capacity		$ 300,000,000		$ 300,000,000
Revolving credit facility present accordion provisions		75,000,000		75,000,000
Accumulated Amortization, Deferred Finance Costs		$ 4,366,000		4,366,000		$ 4,138,000
Amortization of Debt Issuance Costs				$ 614,000	$ 622,000
Houston [Member]
Debt Covenant To Actual Ratios [Line Items]
Deferred payments	$ 2,000,000
Term Number Two [Member]
Debt Covenant To Actual Ratios [Line Items]
Ratio of indebtedness to net capital (in Ratio) (below at March 31, 2015)		3.5		3.5
Ratio Minimum Per Covenant [Member]
Debt Covenant To Actual Ratios [Line Items]
Ratio of earnings to fixed cost obligations (in Ratio)		1.2		1.2
Ratio Actual [Member]
Debt Covenant To Actual Ratios [Line Items]
Ratio of indebtedness to net capital (in Ratio) (below at March 31, 2015)		2.99		2.99
Ratio of earnings to fixed cost obligations (in Ratio)		1.89		1.89
Revolving Credit Facility [Member]
Debt Covenant To Actual Ratios [Line Items]
Revolving credit facility, maximum borrowing capacity		$ 150,000,000		$ 150,000,000
Term Loan [Member]
Debt Covenant To Actual Ratios [Line Items]
Revolving credit facility, maximum borrowing capacity		150,000,000		150,000,000
Credit Agreement [Member]
Debt Covenant To Actual Ratios [Line Items]
Amortization of Debt Issuance Costs		100,000	$ 100,000	200,000	$ 300,000
Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Covenant To Actual Ratios [Line Items]
Amount drawn under term loan facility		$ 75,500,000		$ 75,500,000
Credit Agreement [Member] | Revolving Credit Facility [Member] | QTD [Member]
Debt Covenant To Actual Ratios [Line Items]
Weighted average interest rate		3.40%		3.10%
Credit Agreement [Member] | Term Loan [Member]
Debt Covenant To Actual Ratios [Line Items]
Amount drawn under term loan facility		$ 130,300,000		$ 130,300,000
Debt Instrument, Basis Spread on Variable Rate				2.125%
Prime Rate Option [Member] | Line of Credit [Member]
Debt Covenant To Actual Ratios [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.125%
Libor Margin Option [Member] | Line of Credit [Member]
Debt Covenant To Actual Ratios [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.125%