Note 3 - Investment Securities Available-for-sale - Securities in Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale securities in a continuous loss position, number of securities	148	157
Available-for-sale securities in a continuous loss position, less than 12 months, estimated fair value	$ 331,723	$ 361,079
Available-for-sale securities in a continuous loss position, less than 12 months, unrealized losses	4,280	5,285
Available-for-sale securities in a continuous loss position, 12 months or greater, estimated fair value	24,372	23,170
Available-for-sale securities in a continuous loss position, 12 months or greater, unrealized losses	612	619
Available-for-sale securities in a continuous loss position, estimated fair value	356,095	384,249
Available-for-sale securities in a continuous loss position, unrealized losses	$ 4,892	$ 5,904
US Government Agencies Debt Securities [Member]
Available-for-sale securities in a continuous loss position, number of securities	30	27
Available-for-sale securities in a continuous loss position, less than 12 months, estimated fair value	$ 98,493	$ 88,991
Available-for-sale securities in a continuous loss position, less than 12 months, unrealized losses	1,535	1,764
Available-for-sale securities in a continuous loss position, 12 months or greater, estimated fair value	3,661	3,768
Available-for-sale securities in a continuous loss position, 12 months or greater, unrealized losses	32	38
Available-for-sale securities in a continuous loss position, estimated fair value	102,154	92,759
Available-for-sale securities in a continuous loss position, unrealized losses	$ 1,567	$ 1,802
Residential Mortgage Backed Securities [Member]
Available-for-sale securities in a continuous loss position, number of securities	114	112
Available-for-sale securities in a continuous loss position, less than 12 months, estimated fair value	$ 229,743	$ 232,347
Available-for-sale securities in a continuous loss position, less than 12 months, unrealized losses	2,730	3,110
Available-for-sale securities in a continuous loss position, 12 months or greater, estimated fair value	20,711	19,402
Available-for-sale securities in a continuous loss position, 12 months or greater, unrealized losses	580	581
Available-for-sale securities in a continuous loss position, estimated fair value	250,454	251,749
Available-for-sale securities in a continuous loss position, unrealized losses	$ 3,310	$ 3,691
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities in a continuous loss position, number of securities	4	16
Available-for-sale securities in a continuous loss position, less than 12 months, estimated fair value	$ 3,487	$ 34,743
Available-for-sale securities in a continuous loss position, less than 12 months, unrealized losses	15	400
Available-for-sale securities in a continuous loss position, 12 months or greater, estimated fair value
Available-for-sale securities in a continuous loss position, 12 months or greater, unrealized losses
Available-for-sale securities in a continuous loss position, estimated fair value	3,487	34,743
Available-for-sale securities in a continuous loss position, unrealized losses	$ 15	$ 400
Corporate Debt Securities [Member]
Available-for-sale securities in a continuous loss position, number of securities		2
Available-for-sale securities in a continuous loss position, less than 12 months, estimated fair value		$ 4,998
Available-for-sale securities in a continuous loss position, less than 12 months, unrealized losses		11
Available-for-sale securities in a continuous loss position, 12 months or greater, estimated fair value
Available-for-sale securities in a continuous loss position, 12 months or greater, unrealized losses
Available-for-sale securities in a continuous loss position, estimated fair value		4,998
Available-for-sale securities in a continuous loss position, unrealized losses		$ 11