Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Interest receivable	$ 7,147	$ 4,962
Notes receivable issuances	10,281	10,792
Common shares tendered for tax withholding	343	289
Property liabilities	2,031	3,498
Restatement Adjustment | Reclassification Of Earned Interest Income
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Interest receivable		5,000	$ 7,100	$ 3,200	$ 100
Notes receivable issuances		(5,000)	(7,100)	(3,200)	(100)
Restatement Adjustment | Reclassification For Shares Tendered For Tax Withholding
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Common shares tendered for tax withholding		$ 300	300	200	300
Property liabilities	$ (300)		$ (300)	$ (200)	$ (300)