Loans and Leases - Additional Information (Detail)			3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 25, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) guarantors	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income, Acquired					$ 538,780,000	$ 895,054,000
Carrying amount					1,583,000	7,352,000
Value of buyout for buyout leases					1.00
Impaired Financing Receivable, Recorded Investment					$ 28,482,000
Measure taken to reduce the risk of difference in actual value and residual value of the leased assets at maturity					The Corporation greatly reduces this risk primarily by using $1.00 buyout leases, in which the entire cost of the leased equipment is included in the contractual payments, leaving no residual payment at the end of the lease term.
Charge-offs					$ 7,137,000	6,988,000	$ 8,496,000
Nonaccrual Loans and Leases					14,517,000	17,916,000
Provision for loan losses					9,892,000	4,646,000	3,623,000
Impaired Financing Receivable, Related Allowance					131,000	235,000
Other accruing impaired loans					4,100,000	23,300,000
Specific reserves for other accruing impaired loans					$ 99,000	84,000
Maximum modification period on short-term basis					1 year
Restructured loan past due period					90 days
Minimum [Member]
Financing Receivable, Recorded Investment [Line Items]
Amount of Loan Relationship Balance, Loans Reviewed Annually					$ 1,000,000
Amount of Loan Relationship Balance, Reviewed Quarterly					15,000,000
Maximum [Member]
Financing Receivable, Recorded Investment [Line Items]
Amount of Loan Relationship Balance, Loans Reviewed on a Performance Basis					1,000,000
Fox Chase Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income, Acquired					424,100,000
Carrying amount					792,000
Valley Green Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income, Acquired					114,700,000
Carrying amount					791,000
Lease Financings [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income, Acquired					0	0
Impaired Financing Receivable, Recorded Investment					1,300,000	0
Charge-offs					3,992,000	759,000	801,000
Nonaccrual Loans and Leases					1,599,000	536,000
Provision for loan losses					4,130,000	$ 314,000	$ 644,000
Impaired Financing Receivable, Related Allowance					$ 0
Software Lease [Member]
Financing Receivable, Recorded Investment [Line Items]
Charge-offs			$ 2,800,000
Nonaccrual Loans and Leases			5,000,000	$ 5,000,000
Litigation Settlement, Number of Guarantors | guarantors				29
Litigation Settlement, Settlement Agreement Date		September 25, 2017
Litigation Settlement, Amount Awarded from Other Party		$ 1,000,000
Litigation Settlement, Subject to Election, Payment Per Guarantor				$ 43,000
Accounts receivable			$ 1,300,000	1,300,000
Software Lease [Member] | Minimum [Member]
Financing Receivable, Recorded Investment [Line Items]
Litigation Settlement, Not Subject to Election, Payment Per Guarantor				108,000
Software Lease [Member] | Maximum [Member]
Financing Receivable, Recorded Investment [Line Items]
Litigation Settlement, Amount Awarded from Other Party		$ 1,300,000
Litigation Settlement, Not Subject to Election, Payment Per Guarantor				$ 228,000
Scenario, Forecast [Member] | Software Lease [Member]
Financing Receivable, Recorded Investment [Line Items]
Litigation Settlement, Election Process Completion, Date	Mar. 31, 2018