Long-term Debt, Line of Credit and Derivatives - Provisions of the revolving line of credit and accordion (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Aug. 05, 2015
Debt Instrument [Line Items]
Repayments of debt	$ 21,361,000	$ 74,438,000
Proceeds from Lines of Credit	$ 29,861,000	$ 52,000,000
Line of Credit
Debt Instrument [Line Items]
Weighted average interest rate	3.69%
Revolving line of credit
Debt Instrument [Line Items]
Fair value of amount outstanding	$ 38,000,000
Line of credit facility, remaining borrowing capacity	$ 61,300,000
Revolving line of credit | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 100,000,000
Minimum additional borrowing amount			1,000,000
Amount over minimum additional borrowing amount, integral multiples			250,000
Stated interest rate	3.98%
Revolving line of credit | Letter of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			20,000,000
Letters of credit outstanding, amount	$ 700,000
Revolving line of credit | 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
Term Loan | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	60,000,000
Fair value of amount outstanding	$ 59,300,000
Term Loan | Line of Credit | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Interest rate, effective percentage	4.00%