Loans Held for Investment - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	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		58.00%	67.00%
Gains on sales of loans, net		$ 0	$ 2,060,000	$ 0
Tier 1 of government guarantee					97.00%
Tier 2 of government guarantee						98.00%
Tier 3 of government guarantee							100.00%
Estimated weighted average life of student loans		5 years 4 months 24 days	5 years 5 months
Loans currently in repayment status that were subject to interest rate reductions		7.20%	6.40%
Period of loans past due that have accrued interest		90 days
Loans in nonaccrual status		$ 0	$ 0
Percentage of loans concentrated in major states		39.40%	39.90%
Percentage of aggregate outstanding loans held for investment		5.00%
FFELP Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		$ 48,000,000	$ 43,000,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Ninety or more days delinquent		63,000,000	51,000,000
Private Education Loans
Finance Receivable Transferred To Held For Sale [Line Items]
Loans pledged to borrower in custody		$ 3,400,000,000	$ 3,400,000,000
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]
Interest rate reduction		2.00%
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
Gains on sales of loans, net	$ 2,000,000