Significant Accounting Policies - Additional Information (Details) $ in Thousands			6 Months Ended
	May 31, 2020 USD ($)	Jan. 01, 2020	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items]
Percentage of Private Education Loans related to borrowers in school, grace or deferment			26.00%		25.00%
Interest rate offered to borrowers facing financial difficulty			4.00%
Period of forbearance period to be classified as TDR			3 months
Period of forbearance			24 months
Period after grace period for forbearance allowance for loans			9 months
Forbearance period after grace period for loans			6 months
Percentage of loans granted forbearance (as a percentage)			47.00%		50.00%
Personal loans held for investment			$ 21,163,931		$ 24,667,792
Percentage reimbursement on all qualifying default claims period two (as a percentage)			97.00%
Percentage reimbursement on all qualifying default claims period one (as a percentage)			98.00%
Percentage reimbursement on all qualifying default claims period three (as a percentage)			100.00%
Variable rate			105.00%
Gain on sale of Upromise subsidiary	$ 11,000		$ 11,331	$ 0
Accounting Standards Update 2016-13
Financing Receivable, Impaired [Line Items]
Period of deferment		2
Personal Loans
Financing Receivable, Impaired [Line Items]
Personal loans held for investment			$ 609,051		$ 983,643