Income Taxes - Percentage of Pretax Earnings, Differs from Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	(12.10%)	(11.40%)	(10.60%)
Effect of state income tax	0.10%	0.10%	0.10%
ESOP tax deduction	(0.20%)	(0.20%)	(0.20%)
Other	0.10%	0.30%	0.20%
Effective income tax rate	22.90%	23.80%	24.50%