Loans Held for Investment, net - Impaired Loans Segregated by Loans with No Specific Allowance and Loans with Allowance by Class of Loans, Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 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	$ 6.2	$ 6.6
Average carrying amount not considered impaired	$ 6.0	$ 7.7