Long-Term Debt (Narrative) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Amount outstanding under revolving credit facility	$ 60,082,000	$ 70,090,000
2018	82,000
2019	0
2020	0
2021	60,000,000
Thereafter	0
Amended and Restated Revolving Credit Agreement | Bank revolving credit facility | Unsecured Debt
Debt Instrument [Line Items]
Aggregate commitments	250,000,000
Maximum amount of aggregate commitments	50,000,000
Amount outstanding under revolving credit facility	60,000,000	$ 70,000,000
Available borrowings	188,345,000
Amended and Restated Revolving Credit Agreement | Standby Letters of Credit | Unsecured Debt
Debt Instrument [Line Items]
Amount of capacity	$ 1,655,000