Note 6 - Loans and the Allowance for Loan Losses - (Details Textual) - USD ($)	9 Months Ended			12 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017		Dec. 31, 2016
Loans and Leases Receivable, Deferred Income, Total	$ 1,400,000			$ 1,300,000
Loans and Leases Receivable Reclassified from Overdraft	159,000			129,000
Loans Receivable, Unpaid Principal Balance	129,200,000			82,400,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	5,370,000	[1]		696,000	[1]	$ 1,776,000
Loans and Leases Receivable, Net of Deferred Income, Total	1,297,959,000			975,284,000
Impaired Financing Receivable, Interest Income, Accrual Method, Total	186,000		$ 257,000
Financing Receivable, Modifications, Subsequent Default, Recorded Investment	79,000			3,300,000
Minden Bancorp [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	5,400,000
Minden Bancorp [Member] | Performing Financial Instruments [Member]
Loans and Leases Receivable, Deferred Income, Total	2,700,000
Loans Receivable, Fair Value Disclosure	163,300,000
Loans and Leases Receivable, Net of Deferred Income, Total	$ 160,600,000
American Gateway Financial Corporation [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net				696,000
American Gateway Financial Corporation [Member] | Performing Financial Instruments [Member]
Loans and Leases Receivable, Deferred Income, Total				1,800,000
Loans Receivable, Fair Value Disclosure				46,100,000
Loans and Leases Receivable, Net of Deferred Income, Total				$ 47,200,000

[1] Purchased credit impaired loans are evaluated for impairment on an individual basis.