Long-Term Debt and Loan Agreements - Additional Information (Details) - USD ($)		3 Months Ended			6 Months Ended
	May 30, 2014	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt		$ 80,057,000			$ 80,057,000		$ 152,632,000
Long-term Debt		$ 78,654,000			$ 78,654,000		$ 151,036,000
Weighted average interest rate during period		5.82%	4.87%		5.55%	4.99%
Senior Unsecured Note
Debt Instrument [Line Items]
Long-term Debt		$ 78,000,000			$ 78,000,000
Minimum [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 11,000,000			$ 11,000,000
Interest rate		4.67%			4.67%
Maximum [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 40,000,000			$ 40,000,000
Interest rate		5.45%			5.45%
Loan Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit	$ 300,000,000			$ 200,000,000
Loan maturity period	2018-12			2022-03
Remaining amount available under the line of credit		$ 193,500,000			$ 193,500,000
Letters of credit		$ 4,400,000			$ 4,400,000