Loans Held for Investment - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	59.00%	67.00%
Tier 1 of government guarantee	97.00%		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 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
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	2.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate offered to borrowers facing financial difficulty	4.00%