Note 6 - Loans and the Allowance for Loan Losses - (Details Textual) - USD ($)	3 Months Ended			12 Months Ended
	Mar. 31, 2019		Mar. 31, 2018	Dec. 31, 2018		Dec. 31, 2017
Loans and Leases Receivable, Deferred Income, Total	$ 2,100,000			$ 1,700,000
Loans and Leases Receivable Reclassified from Overdraft	299,000			858,000
Loans Receivable, Unpaid Principal Balance	164,200,000			147,000,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	7,985,000	[1]		10,704,000	[1]	$ 696,000
Loans and Leases Receivable, Net of Deferred Income, Total	1,586,182,000			1,528,469,000
Impaired Financing Receivable, Interest Income, Accrual Method, Total	69,000		$ 116,000
Financing Receivable, Troubled Debt Restructuring, Subsequent Default	0			79,000
Minden Bancorp and Richland State Bancorp [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	8,000,000
Minden Bancorp and Richland State Bancorp [Member] | Performing Financial Instruments [Member]
Loans and Leases Receivable, Deferred Income, Total	2,900,000
Loans Receivable, Fair Value Disclosure	288,200,000
Loans and Leases Receivable, Net of Deferred Income, Total	$ 283,100,000
American Gateway Financial Corporation [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net				10,700,000
American Gateway Financial Corporation [Member] | Performing Financial Instruments [Member]
Loans and Leases Receivable, Deferred Income, Total				3,200,000
Loans Receivable, Fair Value Disclosure				334,800,000
Loans and Leases Receivable, Net of Deferred Income, Total				$ 327,300,000

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