Significant Inputs Used To Price Ginnie Mae Mortgage-Backed Securities (Detail) (Recurring Basis, Level 2)	Mar. 31, 2013	Mar. 31, 2012
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Coupon rate	1.60%	1.60%
Duration (in years)	1 year 2 months 12 days
PSA prepayment speed	283	201
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Coupon rate	4.60%	5.50%
Duration (in years)	5 years 1 month 6 days	6 years 1 month 6 days
PSA prepayment speed	453	495