Taxes on Income - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 12, 2016
Income Tax Examination [Line Items]
Deferred income tax (benefit) expense	$ 0	$ 0
Unrealized losses on investments in debt and equity	2,084,000	4,170,000
Postretirement benefit obligations	300,000	300,000
Unrealized losses on derivatives	800,000	1,100,000
Change in tax rate, deferred tax asset, provisional income tax expense	14,500,000
Deferred tax asset	16,645,000	36,125,000
Deferred tax assets, federal net operating loss carryovers		7,700,000
Deferred income taxes	17,899,000	5,370,000	$ 2,231,000
Deferred tax assets, net operating loss carryovers	188,000	352,000
Excess income tax benefits from stock compensation	2,300,000	1,500,000
Deferred tax asset	27,951,000	45,241,000
Income tax benefit	58,246,000	33,074,000	30,273,000
Amortization of low-income housing credit investments reflected as income tax expense	1,700,000	1,600,000	1,900,000
Tax benefits recorded as income tax expense	400,000
Carrying value of investment in affordable housing credits	13,800,000	15,400,000
Deferred tax liabilities, BOLI, taxes and penalties	8,000,000
Federal
Income Tax Examination [Line Items]
Net operating loss carryforwards	900,000
Federal net operating loss	$ 100,000
Minimum
Income Tax Examination [Line Items]
Percentage of tax benefit	50.00%
Penn Liberty
Income Tax Examination [Line Items]
Business combination, deferred tax asset				$ 7,400,000
Decrease in deferred tax asset	$ 900,000
Alternative minimum tax credit carryovers				$ 100,000
Alliance
Income Tax Examination [Line Items]
Deferred tax assets, federal net operating loss carryovers		1,100,000
Deferred tax assets, state net operating loss carryovers		2,600,000
Deferred tax assets, alternative minimum tax credit carryovers (less than)		1,700,000
Deferred income taxes		0
Deferred tax asset valuation allowance		0
Retained earnings prior Alliance bad debt deductions		$ 7,100,000
First National Bank of Wyoming (FNBW)
Income Tax Examination [Line Items]
Business combination, deferred tax asset			3,100,000
Deferred tax asset valuation allowance			0
Deferred tax assets, net operating loss carryovers			1,900,000
First National Bank of Wyoming (FNBW) | Minimum
Income Tax Examination [Line Items]
Alternative minimum tax credit carryovers			$ 300,000
Tax Credit
Income Tax Examination [Line Items]
Affordable housing tax credits	$ 1,600,000