Income Taxes - Summary of Federal Statutory Income Tax Rate Reconciled to Effective Tax Rate (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory rate	21.00%	34.00%
Increase (decrease) resulting from:
Net tax-exempt income	(5.60%)	85.00%
Merger-related expenses	0.00%	(15.00%)
Tax Cuts and Jobs Act of 2017 adjustment to deferred taxes	0.00%	(283.10%)
Income tax return to provision (2017) adjustment	(3.90%)	0.00%
Other, net	0.70%	11.40%
Federal effective income tax rate	12.10%	(167.70%)

Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation Increase Decrease [Abstract]

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Effective Income Tax Rate Reconciliation, Acquisition Related Cost, Percent

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Effective income tax rate reconciliation, income tax return to provision adjustment.

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Effective income tax rate reconciliation, tax cuts and jobs act adjustment to deferred taxes.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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