Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Temporarily impaired securities, less than 12 months, fair value		$ 556,415,000
Temporarily impaired securities, less than 12 months, unrealized losses		3,343,000
Temporarily impaired securities, 12 months or longer, unrealized losses		13,341,000
Temporarily impaired securities, total fair value		1,181,295,000
Temporarily impaired securities, total unrealized losses	$ 35,200,000	16,684,000
Temporarily impaired securities, 12 months or longer, fair value		624,880,000
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	124,463,000	199,823,000
Temporarily impaired securities, less than 12 months, unrealized losses	49,000	62,000
Temporarily impaired securities, 12 months or longer, unrealized losses	92,000	295,000
Temporarily impaired securities, total fair value	174,369,000	249,520,000
Temporarily impaired securities, total unrealized losses	141,000	357,000
Temporarily impaired securities, 12 months or longer, fair value		49,697,000
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	8,189,000	5,711,000
Temporarily impaired securities, less than 12 months, unrealized losses	208,000	70,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	8,189,000	5,711,000
Temporarily impaired securities, total unrealized losses	208,000	70,000
Temporarily impaired securities, 12 months or longer, fair value
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value
Temporarily impaired securities, less than 12 months, unrealized losses
Temporarily impaired securities, 12 months or longer, unrealized losses	16,041,000	9,664,000
Temporarily impaired securities, total fair value	383,959,000	390,336,000
Temporarily impaired securities, total unrealized losses	16,041,000	9,664,000
Temporarily impaired securities, 12 months or longer, fair value	383,959,000	390,336,000
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	465,499,000	342,436,000
Temporarily impaired securities, less than 12 months, unrealized losses	12,349,000	3,147,000
Temporarily impaired securities, 12 months or longer, unrealized losses	6,407,000	3,112,000
Temporarily impaired securities, total fair value	625,984,000	521,053,000
Temporarily impaired securities, total unrealized losses	18,756,000	6,259,000
Temporarily impaired securities, 12 months or longer, fair value	160,485,000	178,617,000
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		1,914,000
Temporarily impaired securities, less than 12 months, unrealized losses		30,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value		1,914,000
Temporarily impaired securities, total unrealized losses		30,000
Temporarily impaired securities, 12 months or longer, fair value
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	1,203,000	1,516,000
Temporarily impaired securities, less than 12 months, unrealized losses	30,000	17,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	1,203,000	1,516,000
Temporarily impaired securities, total unrealized losses	30,000	17,000
Temporarily impaired securities, 12 months or longer, fair value
Total Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	599,354,000
Temporarily impaired securities, less than 12 months, unrealized losses	12,636,000
Temporarily impaired securities, 12 months or longer, unrealized losses	22,540,000
Temporarily impaired securities, total fair value	1,193,704,000
Temporarily impaired securities, total unrealized losses	35,176,000
Temporarily impaired securities, 12 months or longer, fair value	594,350,000
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		5,015,000
Temporarily impaired securities, less than 12 months, unrealized losses		17,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value		5,015,000
Temporarily impaired securities, total unrealized losses		17,000
Temporarily impaired securities, 12 months or longer, fair value
Preferred Stock of Government Sponsored Entities [Member]
Temporarily impaired securities, less than 12 months, fair value	7,480,000
Temporarily impaired securities, less than 12 months, unrealized losses	2,623,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	7,480,000
Temporarily impaired securities, total unrealized losses	2,623,000
Temporarily impaired securities, 12 months or longer, fair value
Mutual Funds [Member]
Temporarily impaired securities, less than 12 months, fair value
Temporarily impaired securities, less than 12 months, unrealized losses
Temporarily impaired securities, 12 months or longer, unrealized losses	148,000	270,000
Temporarily impaired securities, total fair value	6,082,000	6,230,000
Temporarily impaired securities, total unrealized losses	148,000	270,000
Temporarily impaired securities, 12 months or longer, fair value	6,082,000	$ 6,230,000
Other Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	9,569,000
Temporarily impaired securities, less than 12 months, unrealized losses	2,200,000
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	9,569,000
Temporarily impaired securities, total unrealized losses	2,200,000
Temporarily impaired securities, 12 months or longer, fair value
Total Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	17,049,000
Temporarily impaired securities, less than 12 months, unrealized losses	4,823,000
Temporarily impaired securities, 12 months or longer, unrealized losses	148,000
Temporarily impaired securities, total fair value	23,131,000
Temporarily impaired securities, total unrealized losses	4,971,000
Temporarily impaired securities, 12 months or longer, fair value	$ 6,082,000