Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment securities available for sale, amortized cost	$ 1,320,843	$ 1,336,345
Investment securities available for sale, gross unrealized gains	1,150	13,965
Investment securities available for sale, gross unrealized losses	38,933	16,684
Securities available-for-sale	1,283,060	1,333,626
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	199,488	249,877
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	114	357
Securities available-for-sale	199,374	249,520
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	6,721	9,047
Investment securities available for sale, gross unrealized gains		11
Investment securities available for sale, gross unrealized losses	210	70
Securities available-for-sale	6,511	8,988
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	400,000	400,000
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	16,553	9,664
Securities available-for-sale	383,447	390,336
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	648,532	577,987
Investment securities available for sale, gross unrealized gains	130	241
Investment securities available for sale, gross unrealized losses	22,023	6,259
Securities available-for-sale	626,639	571,969
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale, amortized cost		1,944
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses		30
Securities available-for-sale		1,914
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	1,119	1,533
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	33	17
Securities available-for-sale	1,086	1,516
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	64,983	80,007
Investment securities available for sale, gross unrealized gains	1,020	1,291
Investment securities available for sale, gross unrealized losses		17
Securities available-for-sale	$ 66,003	81,281
Mutual Funds [Member]
Investment securities available for sale, amortized cost		6,500
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses		270
Securities available-for-sale		6,230
Preferred Stock of Government Sponsored Entities [Member]
Investment securities available for sale, amortized cost		5,842
Investment securities available for sale, gross unrealized gains		4,260
Investment securities available for sale, gross unrealized losses
Securities available-for-sale		10,102
Other Equity Securities [Member]
Investment securities available for sale, amortized cost		3,608
Investment securities available for sale, gross unrealized gains		8,162
Investment securities available for sale, gross unrealized losses
Securities available-for-sale		$ 11,770