Income Taxes - Percentage of Pretax Earnings, Differs from the Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Rate Reconciliation [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
Reductions in tax rate resulting from interest income exempt from federal income tax	(9.70%)	(9.30%)	(9.20%)
Effect of state income tax	0.10%	0.10%	0.10%
ESOP tax credit	(0.30%)	(0.30%)	(0.30%)
Other	0.20%	0.30%	0.20%
Effective income tax rate	25.30%	25.80%	25.80%