Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net loss	$ (80,805)	$ (41,978)	$ (15,002)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	9,230	8,465	7,176
Stock-based compensation	27,814	20,438	11,101
Amortization of debt discount and issuance costs	6,385	2,584	0
Non-cash lease expense	6,940
Other	(663)	0	0
Change in assets and liabilities:
Accounts receivable, net	(3,861)	(2,029)	(2,709)
Inventory	(51,896)	(19,312)	(3,382)
Prepaid expenses and other assets	(3,539)	(5,725)	4,450
Accounts payable	(394)	617	(252)
Accrued liabilities and other payables	7,459	4,509	5,115
Lease liabilities	(7,209)
Deferred rent	1	(1,249)	749
Origination of loans held for sale	(395,638)	(86,023)	(11,008)
Proceeds from sale of loans originated as held for sale	378,566	83,001	9,117
Net cash (used in) provided by operating activities	(107,610)	(36,702)	5,355
Investing activities
Purchases of property and equipment	(15,533)	(8,303)	(12,113)
Purchases of investments	(136,265)	(2,000)	(992)
Sales of investments	11,486	0	2,741
Maturities of investments	24,400	0	0
Net cash used in investing activities	(115,912)	(10,303)	(10,364)
Financing activities
Proceeds from the issuance of shares resulting from employee equity plans	16,107	23,407	3,003
Tax payments related to net share settlements on restricted stock units	(5,126)	(1,426)	0
Borrowings from warehouse credit facilities	388,586	83,842	10,746
Repayments of warehouse credit facilities	(372,017)	(81,125)	(8,730)
Borrowings from secured revolving credit facility	4,444	0	0
Other payables - deposits held in escrow	883	2,158	273
Proceeds from issuance of convertible notes, net of issuance costs	0	138,953	0
Proceeds from initial public offering, net of underwriting discounts	0	0	148,088
Payment of initial public offering costs	0	0	(3,558)
Proceeds from follow on offering	0	107,593	0
Cash paid for debt issuance costs	(922)	0	0
Principal payments under finance lease obligations	(72)
Net cash provided by financing activities	31,883	273,402	149,822
Effect of exchange rate changes on cash and cash equivalents	32	0	0
Net change in cash, cash equivalents, and restricted cash	(191,607)	226,397	144,813
Cash, cash equivalents, and restricted cash:
Beginning of period	439,055	212,658	67,845
End of period	247,448	439,055	212,658
Supplemental disclosure of non-cash investing and financial activities
Cash paid for interest	2,460	0	0
Conversion of redeemable convertible preferred stock to common stock	0	0	831,331
Accretion of redeemable convertible preferred stock	0	0	(175,915)
Stock-based compensation capitalized in property and equipment	(1,280)	(522)	(268)
Leasehold improvements paid directly by lessor	$ (6,230)	$ (1,980)	$ (822)