Note 2 - Securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized cost	$ 556,947	$ 571,936
Unrealized Gains	2,508	2,264
Unrealized Losses	3,311	4,126
Fair value	556,144	570,074
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)	556,144	570,074
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost	340,122	268,784
Unrealized Gains	900	800
Unrealized Losses	1,076	1,494
Fair value	339,946	268,090
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)	339,946	268,090
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Amortized cost	161,867	170,344
Unrealized Gains	728	735
Unrealized Losses	2,037	2,236
Fair value	160,558	168,843
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)	160,558	168,843
US States and Political Subdivisions Debt Securities [Member]
Amortized cost	54,305	57,158
Unrealized Gains	757	682
Unrealized Losses	198	396
Fair value	54,864	57,444
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)	54,864	57,444
Common Stock [Member]
Amortized cost	653	653
Unrealized Gains	123	46
Unrealized Losses
Fair value	776	699
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)	$ 776	699
US Treasury Securities [Member]
Amortized cost		74,997
Unrealized Gains		1
Unrealized Losses
Fair value		74,998
Securities available-for-sale (amortized cost of $556,947 in 2017 and $571,936 in 2016)		$ 74,998