Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Temporarily impaired securities, less than 12 months, fair value	$ 1,107,943,000	$ 1,017,655,000
Temporarily impaired securities, less than 12 months, unrealized losses	8,809,000	13,814,000
Temporarily impaired securities, 12 months or longer, fair value	56,122,000	35,458,000
Temporarily impaired securities, 12 months or longer, unrealized losses	491,000	1,154,000
Temporarily impaired securities, total fair value	1,164,065,000	1,053,113,000
Temporarily impaired securities, total unrealized losses	9,300,000	14,968,000
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	339,707,000	299,088,000
Temporarily impaired securities, less than 12 months, unrealized losses	46,000	857,000
Temporarily impaired securities, 12 months or longer, fair value	49,729,000
Temporarily impaired securities, 12 months or longer, unrealized losses	259,000
Temporarily impaired securities, total fair value	389,436,000	299,088,000
Temporarily impaired securities, total unrealized losses	305,000	857,000
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	395,774,000	390,331,000
Temporarily impaired securities, less than 12 months, unrealized losses	6,284,000	9,669,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	395,774,000	390,331,000
Temporarily impaired securities, total unrealized losses	6,284,000	9,669,000
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	1,931,000
Temporarily impaired securities, less than 12 months, unrealized losses	12,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	1,931,000
Temporarily impaired securities, total unrealized losses	12,000
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	359,112,000	328,236,000
Temporarily impaired securities, less than 12 months, unrealized losses	2,359,000	3,288,000
Temporarily impaired securities, 12 months or longer, fair value	122,000	62,000
Temporarily impaired securities, 12 months or longer, unrealized losses	3,000	2,000
Temporarily impaired securities, total fair value	359,234,000	328,298,000
Temporarily impaired securities, total unrealized losses	2,362,000	3,290,000
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	1,710,000
Temporarily impaired securities, less than 12 months, unrealized losses	5,000
Temporarily impaired securities, 12 months or longer, fair value		28,000
Temporarily impaired securities, 12 months or longer, unrealized losses		20,000
Temporarily impaired securities, total fair value	1,710,000	28,000
Temporarily impaired securities, total unrealized losses	5,000	20,000
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	5,029,000
Temporarily impaired securities, less than 12 months, unrealized losses	5,000
Temporarily impaired securities, 12 months or longer, fair value		29,138,000
Temporarily impaired securities, 12 months or longer, unrealized losses		862,000
Temporarily impaired securities, total fair value	5,029,000	29,138,000
Temporarily impaired securities, total unrealized losses	5,000	862,000
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	6,271,000	6,230,000
Temporarily impaired securities, 12 months or longer, unrealized losses	229,000	270,000
Temporarily impaired securities, total fair value	6,271,000	6,230,000
Temporarily impaired securities, total unrealized losses	229,000	$ 270,000
Other Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	4,680,000
Temporarily impaired securities, less than 12 months, unrealized losses	98,000
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	4,680,000
Temporarily impaired securities, total unrealized losses	$ 98,000