Commitments and Contingencies - Additional Information (Detail) $ in Thousands			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 ($)	Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items]
Off-balance Sheet Risks, Amount, Liability					$ 390	$ 385
Reserve for sold mortgages					293
Charge-offs					7,137	6,988	$ 8,496
Nonaccrual Loans and Leases					14,517	$ 17,916
Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Current carrying amount of the contingent obligation					$ 278
Minimum [Member]
Commitments And Contingencies [Line Items]
Term of letter of credit					1 year
Maximum [Member]
Commitments And Contingencies [Line Items]
Term of letter of credit					5 years
Maximum [Member] | Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Off-balance Sheet Risks, Amount, Liability					$ 53,000
Software Lease [Member]
Commitments And Contingencies [Line Items]
Charge-offs			$ 2,800
Nonaccrual Loans and Leases			5,000	$ 5,000
Litigation Settlement, Number of Guarantors | guarantors				29
Litigation Settlement, Settlement Agreement Date		September 25, 2017
Litigation Settlement, Amount Awarded from Other Party		$ 1,000
Litigation Settlement, Subject to Election, Payment Per Guarantor				$ 43
Accounts receivable			$ 1,300	1,300
Software Lease [Member] | Minimum [Member]
Commitments And Contingencies [Line Items]
Litigation Settlement, Not Subject to Election, Payment Per Guarantor				108
Software Lease [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Litigation Settlement, Amount Awarded from Other Party		$ 1,300
Litigation Settlement, Not Subject to Election, Payment Per Guarantor				$ 228
Scenario, Forecast [Member] | Software Lease [Member]
Commitments And Contingencies [Line Items]
Litigation Settlement, Election Process Completion, Date	Mar. 31, 2018