Note 6 - Investment Securities - Investment Securities (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016		Mar. 31, 2016
Securities available-for-sale, amortized cost	$ 1,230,785,000	$ 1,317,012,000
Securities available-for-sale, gross unrealized gains	9,098,000	12,301,000
Securities available-for-sale, gross unrealized losses	12,154,000	14,968,000
Securities available-for-sale	1,227,729,000	1,314,345,000	[1]	$ 1,300,000,000
US Treasury Securities [Member]
Securities available-for-sale, amortized cost	419,881,000	489,839,000
Securities available-for-sale, gross unrealized gains		35,000
Securities available-for-sale, gross unrealized losses	411,000	857,000
Securities available-for-sale	419,470,000	489,017,000
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, amortized cost	400,000,000	400,000,000
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	8,014,000	9,669,000
Securities available-for-sale	391,986,000	390,331,000
Collateralized Mortgage Backed Securities [Member]
Securities available-for-sale, amortized cost	323,017,000	339,241,000
Securities available-for-sale, gross unrealized gains	268,000	309,000
Securities available-for-sale, gross unrealized losses	3,124,000	3,290,000
Securities available-for-sale	320,161,000	336,260,000
Corporate Debt Securities [Member]
Securities available-for-sale, amortized cost	74,968,000	74,965,000
Securities available-for-sale, gross unrealized gains	378,000	247,000
Securities available-for-sale, gross unrealized losses	335,000	862,000
Securities available-for-sale	75,011,000	74,350,000
Collateralized Mortgage Obligations [Member]
Securities available-for-sale, amortized cost		48,000
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses		20,000
Securities available-for-sale		28,000
Mutual Funds [Member]
Securities available-for-sale, amortized cost	6,500,000	6,500,000
Securities available-for-sale, gross unrealized gains
Securities available-for-sale, gross unrealized losses	270,000	270,000
Securities available-for-sale	6,230,000	6,230,000
Preferred Stock of Government Sponsored Entities [Member]
Securities available-for-sale, amortized cost	2,811,000	2,811,000
Securities available-for-sale, gross unrealized gains	3,167,000	4,497,000
Securities available-for-sale, gross unrealized losses
Securities available-for-sale	5,978,000	7,308,000	[2]
Other Equity Securities [Member]
Securities available-for-sale, amortized cost	3,608,000	3,608,000
Securities available-for-sale, gross unrealized gains	5,285,000	7,213,000
Securities available-for-sale, gross unrealized losses
Securities available-for-sale	$ 8,892,000	$ 10,821,000	[2]

[1] Preferred stock of government sponsored entities and other equity securities as of December 31, 2015 were reclassified as level 1 rather than level 2 as originally classified due to the availability of quoted prices in active markets.

[2] Preferred stock of government sponsored entities and other equity securities as of December 31, 2015 were reclassified as level 1 rather than level 2 as originally classified due to the availability of quoted prices in active markets.