Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($)	Mar. 31, 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, fair value		624,880,000
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	$ 31,700,000	16,684,000
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	74,829,000	199,823,000
Temporarily impaired securities, less than 12 months, unrealized losses	19,000	62,000
Temporarily impaired securities, 12 months or longer, fair value	49,766,000	49,697,000
Temporarily impaired securities, 12 months or longer, unrealized losses	228,000	295,000
Temporarily impaired securities, total fair value	124,595,000	249,520,000
Temporarily impaired securities, total unrealized losses	247,000	357,000
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	8,695,000	5,711,000
Temporarily impaired securities, less than 12 months, unrealized losses	99,000	70,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	8,695,000	5,711,000
Temporarily impaired securities, total unrealized losses	99,000	70,000
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, fair value	385,920,000	390,336,000
Temporarily impaired securities, 12 months or longer, unrealized losses	14,080,000	9,664,000
Temporarily impaired securities, total fair value	385,920,000	390,336,000
Temporarily impaired securities, total unrealized losses	14,080,000	9,664,000
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	903,000	1,914,000
Temporarily impaired securities, less than 12 months, unrealized losses	20,000	30,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	903,000	1,914,000
Temporarily impaired securities, total unrealized losses	20,000	30,000
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	437,818,000	342,436,000
Temporarily impaired securities, less than 12 months, unrealized losses	11,042,000	3,147,000
Temporarily impaired securities, 12 months or longer, fair value	167,958,000	178,617,000
Temporarily impaired securities, 12 months or longer, unrealized losses	6,152,000	3,112,000
Temporarily impaired securities, total fair value	605,776,000	521,053,000
Temporarily impaired securities, total unrealized losses	17,194,000	6,259,000
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	1,371,000	1,516,000
Temporarily impaired securities, less than 12 months, unrealized losses	27,000	17,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	1,371,000	1,516,000
Temporarily impaired securities, total unrealized losses	27,000	17,000
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	5,003,000	5,015,000
Temporarily impaired securities, less than 12 months, unrealized losses	25,000	17,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	5,003,000	5,015,000
Temporarily impaired securities, total unrealized losses	25,000	17,000
Total Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	528,619,000
Temporarily impaired securities, less than 12 months, unrealized losses	11,232,000
Temporarily impaired securities, 12 months or longer, fair value	603,644,000
Temporarily impaired securities, 12 months or longer, unrealized losses	20,460,000
Temporarily impaired securities, total fair value	1,132,263,000
Temporarily impaired securities, total unrealized losses	31,692,000
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, fair value	6,129,000	6,230,000
Temporarily impaired securities, 12 months or longer, unrealized losses	371,000	270,000
Temporarily impaired securities, total fair value	6,129,000	6,230,000
Temporarily impaired securities, total unrealized losses	371,000	$ 270,000
Preferred Stock of Government Sponsored Entities [Member]
Temporarily impaired securities, less than 12 months, fair value	6,691,000
Temporarily impaired securities, less than 12 months, unrealized losses	3,411,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	6,691,000
Temporarily impaired securities, total unrealized losses	3,411,000
Other Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	11,334,000
Temporarily impaired securities, less than 12 months, unrealized losses	436,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	11,334,000
Temporarily impaired securities, total unrealized losses	436,000
Total Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	18,025,000
Temporarily impaired securities, less than 12 months, unrealized losses	3,847,000
Temporarily impaired securities, 12 months or longer, fair value	6,129,000
Temporarily impaired securities, 12 months or longer, unrealized losses	371,000
Temporarily impaired securities, total fair value	24,154,000
Temporarily impaired securities, total unrealized losses	$ 4,218,000