Note 4 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities available-for-sale, amortized cost	$ 1,336,345	$ 1,317,012
Securities available-for-sale, gross unrealized gains	13,965	12,301
Securities available-for-sale, gross unrealized losses	16,684	14,968
Securities available-for-sale	1,333,626	1,314,345
US Treasury Securities [Member]
Securities available-for-sale, amortized cost	249,877	489,839
Securities available-for-sale, gross unrealized gains		35
Securities available-for-sale, gross unrealized losses	357	857
Securities available-for-sale	249,520	489,017
US Government Corporations and Agencies Securities [Member]
Securities available-for-sale, amortized cost	9,047
Securities available-for-sale, gross unrealized gains	11
Securities available-for-sale, gross unrealized losses	70
Securities available-for-sale	8,988
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, amortized cost	400,000	400,000
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	9,664	9,669
Securities available-for-sale	390,336	390,331
Collateralized Mortgage Backed Securities [Member]
Securities available-for-sale, amortized cost	577,987	339,241
Securities available-for-sale, gross unrealized gains	241	309
Securities available-for-sale, gross unrealized losses	6,259	3,290
Securities available-for-sale	571,969	336,260
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, amortized cost	1,944
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	30
Securities available-for-sale	1,914
Collateralized Mortgage Obligations [Member]
Securities available-for-sale, amortized cost	1,533	48
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	17	20
Securities available-for-sale	1,516	28
Corporate Debt Securities [Member]
Securities available-for-sale, amortized cost	80,007	74,965
Securities available-for-sale, gross unrealized gains	1,291	247
Securities available-for-sale, gross unrealized losses	17	862
Securities available-for-sale	81,281	74,350
Mutual Funds [Member]
Securities available-for-sale, amortized cost	6,500	6,500
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	270	270
Securities available-for-sale	6,230	6,230
Preferred Stock of Government Sponsored Entities [Member]
Securities available-for-sale, amortized cost	5,842	2,811
Securities available-for-sale, gross unrealized gains	4,260	4,497
Securities available-for-sale, gross unrealized losses
Securities available-for-sale	10,102	7,308
Other Equity Securities [Member]
Securities available-for-sale, amortized cost	3,608	3,608
Securities available-for-sale, gross unrealized gains	8,162	7,213
Securities available-for-sale, gross unrealized losses
Securities available-for-sale	$ 11,770	$ 10,821