Fair Value Measurements - Assets and Liabilities, Not at FV (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Purchased future receivables, net	$ 49,150	$ 43,265
Liabilities:
Senior secured notes, net	179,387	179,289
Carrying Amount
Assets:
Loans, held-for-investment	4,144,416	4,033,972
Purchased future receivables, net	49,150	43,265
Servicing rights	31,480	30,795
Total assets	4,225,046	4,108,032
Liabilities:
Secured borrowings	1,698,937	1,189,392
Securitized debt obligations of consolidated VIEs	1,692,074	1,815,154
Senior secured notes, net	179,387	179,289
Guaranteed loan financing	457,032	485,461
Convertible note, net	111,310	111,040
Corporate debt, net	150,074	149,986
Total liabilities	4,288,814	3,930,322
Carrying Amount | Level 3 inputs
Assets:
Due from servicers and accrued interest	16,000	20,700
Receivable from third parties	600	1,200
Liabilities:
Payable to related parties and accrued interest payable	26,400	20,000
Fair Value | Level 3 inputs
Assets:
Loans, held-for-investment	4,073,947	4,147,831
Purchased future receivables, net	49,150	43,265
Servicing rights	35,006	34,723
Total assets	4,158,103	4,225,819
Liabilities:
Secured borrowings	1,698,937	1,189,392
Securitized debt obligations of consolidated VIEs	1,665,922	1,859,047
Senior secured notes, net	181,708	190,923
Guaranteed loan financing	472,466	515,182
Convertible note, net	67,720	116,654
Corporate debt, net	95,109	161,098
Total liabilities	$ 4,181,862	$ 4,032,296