Loans Held for Investment - Additional Information (Details) - USD ($)	3 Months Ended					12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items]
Private loans indexed to LIBOR	55.00%					55.00%	58.00%
Tier 1 of government guarantee										97.00%
Tier 2 of government guarantee											98.00%
Tier 3 of government guarantee												100.00%
Gains on sales of loans, net	$ (247,000)	$ (4,000)	$ (369,000)	$ 238,935,000		$ 238,315,000	$ 0	$ 2,060,000
Proceeds from sale of loans receivable				3,100,000,000
Net proceeds from sales of loans held for investment, principal				2,900,000,000
Net proceeds from sales of loans held for investment, capitalized interest				199,000,000
Net proceeds from sales of loans held for investment, accrued interest				12,000,000
Net proceeds from sales of personal loans, principal		697,000,000
Net proceeds from personal loans, accrued interest		7,000,000
Loan sale reduction to provision						$ (204,709,000)
Estimated weighted average life of student loans						5 years 4 months 24 days	5 years 4 months 24 days
Interest rate reduction						4.00%
Interest rate offered to borrowers facing financial difficulty, period						2 years
Loans currently in repayment status that were subject to interest rate reductions	7.80%					7.80%	7.20%
Period of loans past due that have accrued interest						90 days
Loans in nonaccrual status						$ 0
Percentage of loans concentrated in major states	38.80%					38.80%	39.40%
Percentage of aggregate outstanding loans held for investment						5.00%
Allowance for credit losses on loans and leases	$ 1,361,723,000					$ 1,361,723,000	$ 441,912,000	341,121,000	$ 251,475,000
Personal Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loan sale reduction to provision		$ 43,000,000				(42,916,000)
Allowance for credit losses on loans and leases	0					0	65,877,000	$ 62,201,000	$ 6,628,000
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	36,000,000					36,000,000	48,000,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent	81,000,000					81,000,000	63,000,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to borrower in custody	3,300,000,000					3,300,000,000	3,400,000,000
Allowance for credit losses on loans and leases	1,355,844,000					1,355,844,000	$ 374,300,000
Private Education Loan, Extended Loan
Finance Receivable Transferred To Held For Sale [Line Items]
Allowance for credit losses on loans and leases	$ 1,000,000,000.0					$ 1,000,000,000.0
Maximum
Finance Receivable Transferred To Held For Sale [Line Items]
Interest rate reduction						4.00%
Increase to life of loan defaults						14.00%
Minimum
Finance Receivable Transferred To Held For Sale [Line Items]
Increase to life of loan defaults						4.00%
Discontinued Operations, Disposed of by Sale
Finance Receivable Transferred To Held For Sale [Line Items]
Split Loans portfolio sales price				$ 43,000,000	$ 43,000,000
Gains on sales of loans, net					$ 2,000,000