Aggregate fair value measurements of assets (Detail) (USD $)	Mar. 31, 2012		Mar. 31, 2011
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	3,229,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	375,794,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	50,000,000	[1]	19,000,000	[1]
Loans held-for-sale	8,000,000	[2]	10,000,000	[2]
Other real estate owned	77,000,000	[3]	13,000,000	[3]
Other Repossessed Assets	0	[3]	0	[3]
Total	135,000,000		42,000,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	[1]	0	[1]
Loans held-for-sale	0	[2]	0	[2]
Other real estate owned	0	[3]	0	[3]
Other Repossessed Assets	0	[3]	0	[3]
Total	0		0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	[1]	0	[1]
Loans held-for-sale	0	[2]	0	[2]
Other real estate owned	0	[3]	0	[3]
Other Repossessed Assets	0	[3]	0	[3]
Total	0		0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	50,000,000	[1]	19,000,000	[1]
Loans held-for-sale	8,000,000	[2]	10,000,000	[2]
Other real estate owned	77,000,000	[3]	13,000,000	[3]
Other Repossessed Assets	0	[3]	0	[3]
Total	135,000,000		42,000,000
Fair Value, Measurements, Nonrecurring [Member] | Unrealized Gain (Loss or Write-down) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	(11,000,000)	[1]	(3,000,000)	[1]
Loans held-for-sale	(1,000,000)	[2]	(1,000,000)	[2]
Other real estate owned	(13,000,000)	[3]	(4,000,000)	[3]
Other Repossessed Assets	0	[3]	0	[3]
Total	$ (25,000,000)		$ (8,000,000)

[1] Relates mostly to certain impaired collateral dependent loans. The impairment was measured based on the fair value of the collateral, which is derived from appraisals that take into consideration prices in observed transactions involving similar assets in similar locations, in accordance with the provisions of ASC Section 310-10-35.

[2] Relates to lower of cost or fair value adjustments on loans held-for-sale and loans transferred from loans held-in-portfolio to loans held-for-sale.

[3] Represents the fair value of foreclosed real estate and other collateral owned that were written down to their fair value. Costs to sell excluded from the reported fair value amount were $5 million at March 31, 2012 and $1 million at March 31, 2011.