Credit Facilities and Long-Term Debt, Line of Credit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 250,000
Line of credit facility, additional borrowing capacity	$ 50,000
Maturity date	Oct. 31, 2020
Additional available borrowing capacity	$ 128,500
LIBOR [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Margin on variable rate	1.25%
LIBOR [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Margin on variable rate	1.75%
Alternate Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Margin on variable rate	0.25%
Alternate Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Margin on variable rate	0.75%
Fixed Assets Included in Borrowing Base [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Borrowing base	$ 25,000
Borrowing base percentage	10.00%
Fixed Assets Not Included in Borrowing Base [Member]
Line of Credit Facility [Line Items]
Borrowing base	$ 31,250
Borrowing base percentage	12.50%
General Electric Capital Corporation Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings under credit facility	$ 47,400	$ 56,600
Weighted average interest rate	1.70%	1.80%
General Electric Capital Corporation Credit Facility [Member] | Index Loan [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings under credit facility	$ 3,400	$ 3,600
Weighted average interest rate	3.80%
Average daily loan balance outstanding	$ 4,900	4,400
General Electric Capital Corporation Credit Facility [Member] | Direct Borrowings [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings under credit facility	$ 44,000	$ 53,000
Weighted average interest rate	1.60%	1.70%
Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 10,000
Line of Credit [Member] | Minimum [Member] | Stock Repurchase [Member]
Line of Credit Facility [Line Items]
Borrowing base	$ 25,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Coverage Ratio	0.01
Line of Credit [Member] | Pay Cash dividend [Member]
Line of Credit Facility [Line Items]
Agreement permissions	$ 20,000
Line of Credit [Member] | Stock Repurchase [Member]
Line of Credit Facility [Line Items]
Agreement permissions	20,000
Line of Credit [Member] | The Credit Agreement Also Permits Acquisitions, Permissible Debt, Cash Dividend Payments And Stock Repurchases [Member]
Line of Credit Facility [Line Items]
Agreement permissions	$ 20,000