Income Taxes - Percentage of Pretax Earnings, Differs from Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	35.00%	35.00%	35.00%
Reductions in tax rate resulting from interest income exempt from federal income tax	(10.60%)	(10.50%)	(9.70%)
Effect of state income tax	0.10%		0.10%
ESOP tax credit	(0.20%)	(0.20%)	(0.30%)
Other	0.20%	0.30%	0.20%
Effective income tax rate	24.50%	24.60%	25.30%