Income Taxes - Percentage of Pretax Earnings, Differs from Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	35.00%	35.00%
Restatement of net deferred tax liability due to change in income tax rate	(0.40%)	(5.30%)
Reductions in tax rate resulting from interest income exempt from federal income tax	(5.20%)	(11.50%)	(12.10%)
Effect of state income tax	0.10%	0.10%	0.10%
ESOP tax deduction	(0.10%)	(0.20%)	(0.20%)
Other	0.10%	0.10%	0.10%
Effective income tax rate	15.50%	18.20%	22.90%