Note 12 - Employee Benefits (Details Textual) - USD ($)	6 Months Ended	12 Months Ended
Note 12 - Employee Benefits (Details Textual) - USD ($)	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 19, 2015
Employee Stock Ownership Plan (ESOP), Shares in ESOP				55,728
Employee Stock Ownership Plan (ESOP), Number of Allocated Shares		2,975	2,732
Employee Stock Ownership Plan (ESOP), Number of Committed-to-be-Released Shares		47,235
Employee Stock Ownership Plan (ESOP), Compensation Expense		$ 45,000	$ 29,000
Share Price at Formation Date		$ 10
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 912,000
Share Price		$ 19.30			$ 10
Employee Stock Ownership Plan (ESOP), Fair Value of Shares Subject to Repurchase Obligation		$ 160,000	73,000
Common Stock, Dividends, Per Share, Cash Paid		$ 0
The 2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	97,524
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		60,080
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 14.09	$ 14.09
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options		$ 165,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value		$ 2.87
The 2017 Equity Incentive Plan [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	69,660
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	60,080
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		5 years
Allocated Share-based Compensation Expense		$ 18,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate		0.00%
The 2017 Equity Incentive Plan [Member] \| Restricted Stock Awards, Restricted Stock Units and Unrestricted Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,864
The 2017 Equity Incentive Plan [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	22,931	22,931
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		5 years
Allocated Share-based Compensation Expense		$ 32,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options		291,000
Pension Plan [Member]
Defined Contribution Plan, Cost		$ 34,000	19,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		3.00%
Pentegra Defined Benefit Plan (The Plan) [Member]
Defined Contribution Plan, Cost		$ 73,000	$ 66,000
Defined Benefit Plan, Funded Percentage		97.20%	97.15%
Defined Benefit Plan, Plan Assets, Contributions by Employer		$ 153,200,000	$ 163,100,000
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		5.00%
Former Executive Officers [Member]
Accrued Employee Benefits		$ 160,000	167,000
Defined Contribution Plan, Cost		0	$ 48,000
President [Member]
Accrued Employee Benefits		66,000
Defined Contribution Plan, Cost		$ 22,000
Defined Benefit Plans, Estimated Future Employer Contributions in Next Five Years				$ 150,000
Defined Benefit Plans, Accrued Interest Rate on Funds				4.50%

New Bancorp, Inc. 10-K