Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Interest receivable			$ 4,604	$ 3,053
Notes receivable issuances			5,816	10,783
Common shares tendered for tax withholding			343	289
Property liabilities			$ (1,973)	(2,489)
Restatement Adjustment | Reclassification Of Earned Interest Income
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Interest receivable	$ 2,200	$ 1,400		3,100	$ 7,100	$ 3,200	$ 100
Notes receivable issuances	(2,200)	(1,400)		(3,100)	(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		300	300	200	300
Property liabilities	$ (300)	$ (300)		$ (300)	$ (300)	$ (200)	$ (300)