Loans Held for Investment - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Receivables [Abstract]
Percent of private loans indexed to LIBOR	74.00%		77.00%
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 5 months 5 days	5 years 6 months 5 days