Borrowings - Warehouse Credit Facilities (Details) - USD ($)	6 Months Ended
	Jun. 30, 2024	Apr. 24, 2024	Apr. 23, 2024	Dec. 31, 2023
Line of Credit Facility [Line Items]
Total payments due	$ 919,437,000			$ 1,048,675,000
Revolving Credit Facility | Upstart Auto Warehouse Trust | Variable Interest Entity, Primary Beneficiary
Line of Credit Facility [Line Items]
Maximum borrowing capacity	0
Total payments due	73,927,000			139,483,000
Assets pledged as collateral	$ 218,419,000			278,022,000
Basis spread on variable rate	3.00%
Revolving Credit Facility | Upstart Auto Warehouse Trust 2 | Variable Interest Entity, Primary Beneficiary
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 50,000,000
Total payments due	0			0
Assets pledged as collateral	$ 0			0
Revolving Credit Facility | Upstart Auto Warehouse Trust 2 | Variable Interest Entity, Primary Beneficiary | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility | Upstart Auto Warehouse Trust 2 | Variable Interest Entity, Primary Beneficiary | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%
Revolving Credit Facility | Upstart Loan Trust
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 325,000,000
Total payments due	148,873,000			247,942,000
Assets pledged as collateral	$ 238,105,000			361,195,000
Revolving Credit Facility | Upstart Loan Trust | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.80%
Revolving Credit Facility | Upstart Loan Trust | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	3.80%
Revolving Credit Facility | Upstart Small Dollar Loan Trust
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000
Total payments due	35,387,000			0
Assets pledged as collateral	$ 57,944,000			0
Commitment fee threshold	75.00%
Basis spread on variable rate	5.50%
Revolving Credit Facility | Warehouse credit facilities
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 475,000,000
Total payments due	258,187,000			387,425,000
Assets pledged as collateral	$ 514,468,000			$ 639,217,000
Revolving Credit Facility | Warehouse credit facilities | Minimum
Line of Credit Facility [Line Items]
Unused commitment fee	0.50%
Revolving Credit Facility | Warehouse credit facilities | Maximum
Line of Credit Facility [Line Items]
Unused commitment fee	1.00%
Revolving Credit Facility | ULT Warehouse Credit Facility - UnCommitted
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 150,000,000	$ 150,000,000	$ 75,000,000