Securitizations and Variable Interest Entities - Summary of Personal Whole Loan Securitizations and Sales (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Cash proceeds from loans securitized or sold	$ 42,500	$ 258,767	$ 313,486
Unconsolidated Securitization Trusts
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Principal derecognized from loans securitized or sold	1,553,847	1,300,838
Net gains (losses) recognized from loans securitized or sold	4,809	6,039
Fair value of senior securities and subordinated securities retained upon settlement	75,924	65,653
Cash proceeds from loans securitized or sold	1,212,521	867,875
Cash proceeds from servicing and other administrative fees on loans securitized or sold	16,961	13,725
Cash proceeds for interest received on senior securities and subordinated securities	5,022	3,049
Unconsolidated Certificate Program Trusts
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Principal derecognized from loans securitized or sold	2,868,709	1,145,616
Net gains (losses) recognized from loans securitized or sold	32,417	10,483
Fair value of senior securities and subordinated securities retained upon settlement	140,825	56,764
Cash proceeds from loans securitized or sold	2,555,713	1,088,212
Cash proceeds from servicing and other administrative fees on loans securitized or sold	17,071	3,650
Cash proceeds for interest received on senior securities and subordinated securities	7,717	1,747
Senior Securities | Structured Program Transactions
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Interest retained by the Company	98,700	57,300
CLUB Certificate asset-backed securities | Structured Program Transactions
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Interest retained by the Company	101,300	56,800
Subordinated Certificates | Structured Program Transactions
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Interest retained by the Company	$ 16,800	$ 8,300