Loans Held for Investment and Loans Held for Investment at Fair Value - Schedule of Loans Held for Investment Pledged as Collateral for Warehouse Facility Agreements and Securitizations Issued (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accounts Notes And Loans Receivable [Line Items]
Total pledged loans	$ 268,770	$ 287,524
Total	1,789,259	1,549,610
Citibank Warehouse Repurchase Agreement
Accounts Notes And Loans Receivable [Line Items]
Total pledged loans	148,272	108,504
Barclays Warehouse Repurchase Agreement
Accounts Notes And Loans Receivable [Line Items]
Total pledged loans	120,498	175,689
Pacific Western Bank Agreement
Accounts Notes And Loans Receivable [Line Items]
Total pledged loans		3,331
2014-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	26,124	29,559
2015-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	59,168	64,876
2016-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	82,693	97,727
2016-2 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	56,517	68,961
2017-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	95,491	116,670
2017-2 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	157,558	173,390
2018-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	129,111	141,567
2018-2 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	239,362	260,278
2019-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	218,681	229,151
2019-2 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	193,185	210,312
2019-3 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	146,479	$ 157,119
2020-1 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	256,683
2020-2 Trust
Accounts Notes And Loans Receivable [Line Items]
Total	$ 128,207