Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($)	Sep. 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, 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	$ 38,900,000	16,684,000
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	199,374,000	199,823,000
Temporarily impaired securities, less than 12 months, unrealized losses	114,000	62,000
Temporarily impaired securities, 12 months or longer, fair value		49,697,000
Temporarily impaired securities, 12 months or longer, unrealized losses		295,000
Temporarily impaired securities, total fair value	199,374,000	249,520,000
Temporarily impaired securities, total unrealized losses	114,000	357,000
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	5,208,000	5,711,000
Temporarily impaired securities, less than 12 months, unrealized losses	160,000	70,000
Temporarily impaired securities, 12 months or longer, fair value	1,304,000
Temporarily impaired securities, 12 months or longer, unrealized losses	50,000
Temporarily impaired securities, total fair value	6,512,000	5,711,000
Temporarily impaired securities, total unrealized losses	210,000	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	383,447,000	390,336,000
Temporarily impaired securities, 12 months or longer, unrealized losses	16,553,000	9,664,000
Temporarily impaired securities, total fair value	383,447,000	390,336,000
Temporarily impaired securities, total unrealized losses	16,553,000	9,664,000
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	247,059,000	342,436,000
Temporarily impaired securities, less than 12 months, unrealized losses	6,088,000	3,147,000
Temporarily impaired securities, 12 months or longer, fair value	372,218,000	178,617,000
Temporarily impaired securities, 12 months or longer, unrealized losses	15,935,000	3,112,000
Temporarily impaired securities, total fair value	619,277,000	521,053,000
Temporarily impaired securities, total unrealized losses	22,023,000	6,259,000
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, fair value
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
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value		1,516,000
Temporarily impaired securities, less than 12 months, unrealized losses		17,000
Temporarily impaired securities, 12 months or longer, fair value	1,086,000
Temporarily impaired securities, 12 months or longer, unrealized losses	33,000
Temporarily impaired securities, total fair value	1,086,000	1,516,000
Temporarily impaired securities, total unrealized losses	33,000	17,000
Total Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	451,641,000
Temporarily impaired securities, less than 12 months, unrealized losses	6,362,000
Temporarily impaired securities, 12 months or longer, fair value	758,055,000
Temporarily impaired securities, 12 months or longer, unrealized losses	32,571,000
Temporarily impaired securities, total fair value	1,209,696,000
Temporarily impaired securities, total unrealized losses	38,933,000
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, fair value
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
Preferred Stock of Government Sponsored Entities [Member]
Temporarily impaired securities, less than 12 months, fair value	6,397,000
Temporarily impaired securities, less than 12 months, unrealized losses	3,706,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	6,397,000
Temporarily impaired securities, total unrealized losses	3,706,000
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	6,034,000	6,230,000
Temporarily impaired securities, 12 months or longer, unrealized losses	195,000	270,000
Temporarily impaired securities, total fair value	6,034,000	6,230,000
Temporarily impaired securities, total unrealized losses	195,000	$ 270,000
Other Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	1,008,000
Temporarily impaired securities, less than 12 months, unrealized losses	679,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	1,008,000
Temporarily impaired securities, total unrealized losses	679,000
Total Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	7,405,000
Temporarily impaired securities, less than 12 months, unrealized losses	4,385,000
Temporarily impaired securities, 12 months or longer, fair value	6,034,000
Temporarily impaired securities, 12 months or longer, unrealized losses	195,000
Temporarily impaired securities, total fair value	13,439,000
Temporarily impaired securities, total unrealized losses	$ 4,580,000