Segment Reporting - Reconciliation of Operating Profit (Loss) from Segments to Consolidated (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	$ 140,255	$ 109,479	$ 362,791	$ 274,282
Cost of revenue	97,950	70,166	269,576	191,633
Gross profit	42,305	39,313	93,215	82,649
Operating expenses	39,016	29,066	124,643	96,252
Interest income	1,775	311	3,082	387
Interest expense	1,610	0	1,610	0
Other income, net	21	0	200	13
Net income (loss)	3,475	10,558	(29,756)	(13,203)	$ (15,002)
Real estate services
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	126,265	103,864	332,047	262,894
Properties
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	11,350	3,364	23,388	5,345
Operating Segments | Real estate services
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	126,265	103,864	332,047	262,894
Cost of revenue	83,274	64,258	236,775	178,850
Gross profit	42,991	39,606	95,272	84,044
Operating Segments | Properties
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	11,350	3,364	23,388	5,345
Cost of revenue	11,656	3,326	24,086	5,361
Gross profit	(306)	38	(698)	(16)
Other revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenue	2,640	2,251	7,356	6,043
Cost of revenue	3,020	2,582	8,715	7,422
Gross profit	$ (380)	$ (331)	$ (1,359)	$ (1,379)