Securitizations and Variable Interest Entities - Summary of Personal Whole Loan Securitizations and Sales (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Other assets	[1]	$ 102,396		$ 102,396		$ 156,278
Personal Whole Loan Securitization
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		646,242	$ 336,658	1,001,490	$ 336,658
Net gains (losses) recognized from loans securitized or sold		2,412	1,739	5,509	1,739
Fair value of senior securities and subordinated certificates received		32,291	17,536	50,784	17,536
Cash proceeds from loans securitized or sold		307,094	260,829	590,366	260,829
Cash proceeds from servicing and other administrative fees on loans securitized or sold		3,143	70	5,493	70
Cash proceeds for interest received on senior securities and subordinated certificates		1,006	0	1,302	0
Personal Whole Loan Securitization | Senior securities
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Transferor's Interests Retained in Transferred Financial Assets, Fair Value		28,700		43,800	14,000
Personal Whole Loan Securitization | Subordinated residual certificates
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Transferor's Interests Retained in Transferred Financial Assets, Fair Value		3,600	3,600	7,000	3,600
Personal Whole Loan CLUB Certificates
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		196,670	0	358,545	0
Net gains (losses) recognized from loans securitized or sold		1,580	0	3,037	0
Fair value of senior securities and subordinated certificates received		9,724	0	17,826	0
Cash proceeds from loans securitized or sold		185,966	0	340,805	0
Cash proceeds from servicing and other administrative fees on loans securitized or sold		572	0	707	0
Cash proceeds for interest received on senior securities and subordinated certificates		$ 329	$ 0	$ 411	$ 0

[1] Includes amounts in consolidated variable interest entities (VIEs) presented separately in the table below.