Loans Held for Investment - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 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	55.00%		58.00%
Tier 1 of government guarantee (at least)	97.00%			97.00%
Tier 2 of government guarantee					98.00%
Tier 3 of government guarantee						100.00%
Gain from sale	$ 238,935	$ 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
Estimated weighted average life of student loans	5 years 3 months 18 days		5 years 4 months 24 days
Interest rate offered to borrowers facing financial difficulty	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	8.50%		7.20%
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	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%
Increase to life of loan defaults	14.00%