Debt - Margin for Borrowings under Revolving Credit Facility (Details) - Revolving Credit Facility	12 Months Ended
	Dec. 31, 2017 USD ($)
Minimum | III
Line of Credit Facility [Line Items]
Availability of borrowing	$ 24,000,000
Minimum | II
Line of Credit Facility [Line Items]
Availability of borrowing	12,000,000
Maximum | II
Line of Credit Facility [Line Items]
Availability of borrowing	24,000,000
Maximum | I
Line of Credit Facility [Line Items]
Availability of borrowing	$ 12,000,000
Base Rate Loans | III
Line of Credit Facility [Line Items]
Basis spread (as a percent)	0.50%
Base Rate Loans | II
Line of Credit Facility [Line Items]
Basis spread (as a percent)	0.75%
Base Rate Loans | I
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.00%
LIBOR Revolver Loans | III
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.50%
LIBOR Revolver Loans | II
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.75%
LIBOR Revolver Loans | I
Line of Credit Facility [Line Items]
Basis spread (as a percent)	2.00%