Note 8 - Securities Sold Under Agreements to Repurchase - Maturity of Short and Long-term Repurchase Agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short and long-term purchase agreements	$ 34,252	$ 36,370
Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	34,252	29,188
US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		7,182
Maturity Overnight [Member]
Short and long-term purchase agreements	32,341	33,682
Maturity Overnight [Member] | Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	32,341	28,086
Maturity Overnight [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		5,596
Maturity Less than 30 Days [Member]
Short and long-term purchase agreements	1,200	1,203
Maturity Less than 30 Days [Member] | Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	1,200	902
Maturity Less than 30 Days [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		301
Maturity 30 to 90 Days [Member]
Short and long-term purchase agreements	0	0
Maturity 30 to 90 Days [Member] | Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	0	0
Maturity 30 to 90 Days [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		0
Maturity 90 Days to One Year [Member]
Short and long-term purchase agreements	454	458
Maturity 90 Days to One Year [Member] | Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	454	200
Maturity 90 Days to One Year [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		258
Maturity Over One Year to Four Years [Member]
Short and long-term purchase agreements	257	1,027
Maturity Over One Year to Four Years [Member] | Residential Mortgage Backed Securities [Member]
Short and long-term purchase agreements	$ 257	0
Maturity Over One Year to Four Years [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short and long-term purchase agreements		$ 1,027