Disclosures About Fair Value of Assets and Liabilities - Quantitative Information about Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Jun. 30, 2012	Sep. 30, 2012
Mortgage servicing rights [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Fair value of assets	$ 356	$ 329	$ 329
Valuation Technique	Discounted cash flow	Discounted cash flow
Mortgage servicing rights [Member] | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Input Discount Rate	10.50%	10.50%
Unobservable Inputs Constant prepayment rate	18.90%	16.90%
Unobservable Inputs Probability of default	0.29%	0.29%
Mortgage servicing rights [Member] | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Input Discount Rate	11.50%	11.50%
Unobservable Inputs Constant prepayment rate	24.50%	22.40%
Unobservable Inputs Probability of default	0.32%	0.32%
Mortgage servicing rights [Member] | Weighted Average [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Input Discount Rate	10.50%	10.50%
Unobservable Inputs Constant prepayment rate	23.10%	21.00%
Unobservable Inputs Probability of default	0.32%	0.32%
Collateral-dependent impaired loans [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Fair value of assets	512	2,438
Valuation Technique	Market comparable properties	Market comparable properties
Collateral-dependent impaired loans [Member] | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Marketability discount	0.00%	0.00%
Collateral-dependent impaired loans [Member] | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Marketability discount	24.00%	24.00%
Collateral-dependent impaired loans [Member] | Weighted Average [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Marketability discount	16.00%	15.00%
Foreclosed assets [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Fair value of assets		$ 279
Valuation Technique		Market comparable properties
Foreclosed assets [Member] | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Comparability adjustments		12.00%
Foreclosed assets [Member] | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Comparability adjustments		24.00%
Foreclosed assets [Member] | Weighted Average [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Unobservable Inputs Comparability adjustments		19.00%