Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net loss	$ (18,527)	$ (80,805)	$ (41,978)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	14,564	9,230	8,465
Stock-based compensation	36,973	27,814	20,438
Amortization of debt discount and issuance costs	12,038	6,385	2,584
Non-cash lease expense	9,204	6,940
Impairment costs	2,063	0	0
Loss on repurchases and conversions of convertible senior notes	4,634	0	0
Net gain on IRLCs, forward sales commitments and loans held for sale	(1,921)	(493)	(219)
Other	(349)	(663)	0
Change in assets and liabilities:
Accounts receivable, net	(35,496)	(3,861)	(2,029)
Inventory	25,432	(51,896)	(19,312)
Prepaid expenses and other assets	2,333	(3,293)	(5,500)
Accounts payable	2,086	(394)	617
Accrued liabilities, other payables, and payroll tax liabilities, noncurrent	39,092	7,422	4,435
Lease liabilities	(11,312)	(7,209)
Deferred rent	0	1	(1,249)
Origination of loans held for sale	(677,310)	(395,354)	(85,955)
Proceeds from sale of loans originated as held for sale	657,763	378,566	83,001
Net cash provided by (used in) operating activities	61,267	(107,610)	(36,702)
Investing activities
Purchases of property and equipment	(14,686)	(15,533)	(8,303)
Purchases of investments	(198,172)	(136,265)	(2,000)
Sales of investments	7,887	11,486	0
Maturities of investments	147,852	24,400	0
Net cash used in investing activities	(57,119)	(115,912)	(10,303)
Proceeds from the issuance of convertible preferred stock, net of issuance costs	39,801	0	0
Proceeds from the issuance of common stock, net of issuance costs	69,701	0	107,593
Financing activities
Proceeds from the issuance of common stock pursuant to employee equity plans	21,072	16,107	23,407
Tax payments related to net share settlements on restricted stock units	(16,852)	(5,126)	(1,426)
Borrowings from warehouse credit facilities	662,278	388,586	83,842
Repayments to warehouse credit facilities	(644,551)	(372,017)	(81,125)
Borrowings from secured revolving credit facility	89,619	4,444	0
Repayments of Secured Debt	(70,115)	0	0
Other payables - deposits held in escrow	4,074	883	2,158
Proceeds from issuance of convertible senior notes, net of issuance costs	647,486	0	138,953
Payments for repurchases and conversions of convertible senior notes	(108,061)	0	0
Cash paid for secured revolving credit facility issuance costs	(4)	(922)	0
Principal payments under finance lease obligations	(221)	(72)	0
Net cash provided by financing activities	694,227	31,883	273,402
Effect of exchange rate changes on cash and cash equivalents	(3)	32	0
Net change in cash, cash equivalents, and restricted cash	698,372	(191,607)	226,397
Cash, cash equivalents, and restricted cash:
Beginning of period	247,448	439,055	212,658
End of period	945,820	247,448	439,055
Supplemental disclosure of cash flow information
Cash paid for interest	4,958	2,460	0
Non-cash transactions
Stock-based compensation capitalized in property and equipment	2,348	1,280	522
Property and equipment additions in accounts payable and accrued liabilities	1,682	223	82
Leasehold improvements paid directly by lessor	37	6,230	1,980
Issuance of common stock for repurchases and conversions of convertible senior notes	$ 98,397	$ 0	$ 0