Loans Held for Investment - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Receivables [Abstract]
Percent of private loans indexed to LIBOR	78.20%	81.40%
Tier 1 of government guarantee (in percentage)	97.00%		97.00%
Tier 2 of government guarantee (in percentage)				98.00%
Tier 3 of government guarantee (in percentage)					100.00%
Estimated weighted average life of student loans (in years)	5 years 7 months 3 days	6 years