Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jul. 01, 2006
Receivables [Abstract]
Troubled Debt Restructuring	$ 187.1
Criteria for loans to be considered as nonperforming	90 days
Tier 1 of government guarantee	97.00%		97.00%
TDR payment default period	60 days
Ffelp loans	$ 722.0
Percentage of FFELP rehabilitation loans	61.20%	62.10%
Period of loans past due that have accrued interest	90 days