Loans Held for Investment, net - Impaired Loans Segregated by Loans with No Specific Allowance and Loans with Allowance by Class of Loans (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Receivables [Abstract]
Proportion based allowance for loan losses		$ 0.5
Total value of loans for which an allowance established but individual reviews not completed		$ 1.9
Period for loans deemed as non-accrual to return to accrual status	90 days	90 days
Guaranteed portion of education loans	$ 6.3	$ 6.6
Average carrying amount not considered impaired	$ 6.1	$ 7.7