Loans Held for Investment - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	54.00%			54.00%		58.00%
Tier 1 of government guarantee (at least)	97.00%			97.00%			97.00%
Tier 2 of government guarantee								98.00%
Tier 3 of government guarantee									100.00%
Gain from sale	$ (4)	$ 239,000	$ 0	$ 238,562	$ 0
Proceeds from sale of loans receivable		3,100,000
Net proceeds from sales of loans held for investment, principal		2,900,000
Net proceeds from sales of loans held for investment, capitalized interest		199,000
Net proceeds from sales of loans held for investment, accrued interest		$ 12,000
Net proceeds from sales of personal loans, principal	697,000
Net proceeds from personal loans, accrued interest	7,000
Loan sale reduction to provision	$ (42,916)			$ (204,709)
Estimated weighted average life of student loans				5 years 4 months 24 days		5 years 4 months 24 days
Interest rate offered to borrowers facing financial difficulty	4.00%			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	8.80%			8.80%		7.20%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan sale reduction to provision	$ (42,916)			$ (42,916)
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	2.00%			2.00%
Increase to life of loan defaults				4.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	4.00%			4.00%
Increase to life of loan defaults				14.00%