Other-Than-Temporary Impairment Losses Recognized In Earnings By Year Of Vintage (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Other-than-temporary impairment losses recognized in earnings by year of vintage	$ 1,002	$ 2,391	$ 2,197
Non-agency CMOs | Prior to 2005
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Other-than-temporary impairment losses recognized in earnings by year of vintage	7
Non-agency CMOs | 2005
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Other-than-temporary impairment losses recognized in earnings by year of vintage	330
Non-agency CMOs | 2007
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Other-than-temporary impairment losses recognized in earnings by year of vintage	$ 665