Long-term Debt and Line of Credit - Provisions of the Revolving Line of Credit and Accordion (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 05, 2015
Debt Instrument [Line Items]
Repayments of debt		$ 87,813,000	$ 63,084,000	$ 42,955,000
Proceeds from lines of credit		72,000,000	$ 57,000,000	$ 149,021,000
Line of Credit
Debt Instrument [Line Items]
Stated interest rate					4.75%
Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 10,000,000	10,000,000
Remaining borrowing capacity	39,300,000	39,300,000
Repayments of debt	$ 70,000,000
Proceeds from lines of credit		$ 72,000,000
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 50,000,000
Minimum additional borrowing amount					1,000,000
Amount over minimum additional borrowing amount, integral multiples					250,000
Stated interest rate	5.00%	5.00%
Revolving Credit Facility | Letter of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					20,000,000
Letters of credit outstanding	$ 700,000	$ 700,000
Revolving Credit Facility | Bridge Loan
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					5,000,000
Minimum additional borrowing amount					250,000
Amount over minimum additional borrowing amount, integral multiples					$ 50,000