Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities available-for-sale, amortized cost	$ 1,364,955	$ 1,317,012
Securities available-for-sale, gross unrealized gains	12,832	12,301
Securities available-for-sale, gross unrealized losses	9,300	14,968
Securities available-for-sale	1,368,487	1,314,345
US Treasury Securities [Member]
Securities available-for-sale, amortized cost	399,741	489,839
Securities available-for-sale, gross unrealized gains		35
Securities available-for-sale, gross unrealized losses	305	857
Securities available-for-sale	399,436	489,017
US Government Corporations and Agencies Securities [Member]
Securities available-for-sale, amortized cost	9,679
Securities available-for-sale, gross unrealized gains	27
Securities available-for-sale, gross unrealized losses	6
Securities available-for-sale	9,700
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	6,278	9,669
Securities available-for-sale	393,722	390,331
Collateralized Mortgage Backed Securities [Member]
Securities available-for-sale, amortized cost	447,959	339,241
Securities available-for-sale, gross unrealized gains	468	309
Securities available-for-sale, gross unrealized losses	2,362	3,290
Securities available-for-sale	446,065	336,260
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, amortized cost	1,943
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	12
Securities available-for-sale	1,931
Collateralized Mortgage Obligations [Member]
Securities available-for-sale, amortized cost	1,715	48
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	5	20
Securities available-for-sale	1,710	28
Corporate Debt Securities [Member]
Securities available-for-sale, amortized cost	80,007	74,965
Securities available-for-sale, gross unrealized gains	904	247
Securities available-for-sale, gross unrealized losses	5	862
Securities available-for-sale	80,906	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	229	270
Securities available-for-sale	6,271	6,230
Preferred Stock of Government Sponsored Entities [Member]
Securities available-for-sale, amortized cost	4,117	2,811
Securities available-for-sale, gross unrealized gains	3,970	4,497
Securities available-for-sale, gross unrealized losses
Securities available-for-sale	8,087	7,308
Other Equity Securities [Member]
Securities available-for-sale, amortized cost	13,294	3,608
Securities available-for-sale, gross unrealized gains	7,463	7,213
Securities available-for-sale, gross unrealized losses	98
Securities available-for-sale	$ 20,659	$ 10,821