Condensed Consolidated Statements of Cash Flows - 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
Operating activities:
Net loss	$ 3,475	$ 10,558	$ (29,756)	$ (13,203)	$ (15,002)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization			6,123	5,326
Stock-based compensation			14,472	8,028
Amortization of debt discount and issuance costs			1,128	0
Change in assets and liabilities:
Prepaid expenses			2,680	(84)
Accrued revenue			80	(2,712)
Inventory			(21,779)	(5,399)
Other current assets			(576)	8,556
Other long-term assets			(296)	244
Accounts payable			702	1,228
Accrued liabilities			10,943	8,513
Deferred lease liability			(913)	1,001
Accrued expenses			414	0
Origination of loans held for sale			(56,157)	(5,755)
Proceeds from sale of loans originated as held for sale			52,127	5,030
Net cash provided by (used in) operating activities			(20,808)	10,773
Investing activities:
Maturities and sales of short-term investments			0	1,484
Purchases of short-term investments			0	(993)
Purchases of property and equipment			(5,528)	(10,499)
Net cash used in investing activities			(5,528)	(10,008)
Financing activities:
Proceeds from issuance of convertible senior notes, net			138,953	0
Proceeds from follow-on offering, net			107,593	0
Proceeds from issuance of common stock			17,314	2,519
Tax payment related to net share settlements on restricted stock units			(705)	0
Proceeds from initial public offering, net of underwriting discounts			0	148,088
Payment of initial public offering costs			0	(3,449)
Borrowings from warehouse credit facilities			54,806	5,603
Repayments of warehouse credit facilities			(51,031)	(4,898)
Other payables - customer escrow deposits related to title services			7,684	6,065
Net cash provided by financing activities			274,614	153,928
Net change in cash, cash equivalents, and restricted cash			248,278	154,693
Cash, cash equivalents, and restricted cash:
Beginning of period			212,658	67,845	67,845
End of period	460,936	222,538	460,936	222,538	$ 212,658
Supplemental disclosure of non-cash investing and financing activities:
Accretion of redeemable convertible preferred stock	$ 0	$ (40,224)	0	(175,915)
Stock-based compensation capitalized in property and equipment			(363)	(194)
Initial public offering cost accruals			0	(200)
Property and equipment additions in accounts payable and accrued expenses			(25)	0
Leasehold improvements paid directly by lessor			(926)	(104)
Cash in transit for exercised stock options			$ (85)	$ 0