Note 4 - Investment Securities - Temporarily Impaired Securities (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Temporarily impaired securities, less than 12 months, fair value	$ 556,415	$ 1,017,655
Temporarily impaired securities, less than 12 months, unrealized losses	$ 3,343	$ 13,814
Temporarily impaired securities, less than 12 months, no. Of issuances	63	30
Temporarily impaired securities, 12 months or longer, fair value	$ 624,880	$ 35,458
orarily impaired securities, 12 months or longer, unrealized losses	$ 13,341	$ 1,154
Temporarily impaired securities, 12 months or longer, no. Of issuances	24	7
porarily impaired securities, total fair value	$ 1,181,295	$ 1,053,113
porarily impaired securities, total unrealized losses	$ 16,684	$ 14,968
Temporarily impaired securities, total no. Of issuances	87	37
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 199,823	$ 299,088
Temporarily impaired securities, less than 12 months, unrealized losses	$ 62	$ 857
Temporarily impaired securities, less than 12 months, no. Of issuances	4	6
Temporarily impaired securities, 12 months or longer, fair value	$ 49,697
orarily impaired securities, 12 months or longer, unrealized losses	$ 295
Temporarily impaired securities, 12 months or longer, no. Of issuances	2
porarily impaired securities, total fair value	$ 249,520	$ 299,088
porarily impaired securities, total unrealized losses	$ 357	$ 857
Temporarily impaired securities, total no. Of issuances	6	6
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 5,711
Temporarily impaired securities, less than 12 months, unrealized losses	$ 70
Temporarily impaired securities, less than 12 months, no. Of issuances	3
Temporarily impaired securities, 12 months or longer, fair value
orarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
porarily impaired securities, total fair value	$ 5,711
porarily impaired securities, total unrealized losses	$ 70
Temporarily impaired securities, total no. Of issuances	3
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		$ 390,331
Temporarily impaired securities, less than 12 months, unrealized losses		$ 9,669
Temporarily impaired securities, less than 12 months, no. Of issuances		8
Temporarily impaired securities, 12 months or longer, fair value	$ 390,336
orarily impaired securities, 12 months or longer, unrealized losses	$ 9,664
Temporarily impaired securities, 12 months or longer, no. Of issuances	8
porarily impaired securities, total fair value	$ 390,336	$ 390,331
porarily impaired securities, total unrealized losses	$ 9,664	$ 9,669
Temporarily impaired securities, total no. Of issuances	8	8
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 342,436	$ 328,236
Temporarily impaired securities, less than 12 months, unrealized losses	$ 3,147	$ 3,288
Temporarily impaired securities, less than 12 months, no. Of issuances	48	16
Temporarily impaired securities, 12 months or longer, fair value	$ 178,617	$ 62
orarily impaired securities, 12 months or longer, unrealized losses	$ 3,112	$ 2
Temporarily impaired securities, 12 months or longer, no. Of issuances	13	3
porarily impaired securities, total fair value	$ 521,053	$ 328,298
porarily impaired securities, total unrealized losses	$ 6,259	$ 3,290
Temporarily impaired securities, total no. Of issuances	61	19
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 1,914
Temporarily impaired securities, less than 12 months, unrealized losses	$ 30
Temporarily impaired securities, less than 12 months, no. Of issuances	2
Temporarily impaired securities, 12 months or longer, fair value
orarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
porarily impaired securities, total fair value	$ 1,914
porarily impaired securities, total unrealized losses	$ 30
Temporarily impaired securities, total no. Of issuances	2
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 1,516
Temporarily impaired securities, less than 12 months, unrealized losses	$ 17
Temporarily impaired securities, less than 12 months, no. Of issuances	5
Temporarily impaired securities, 12 months or longer, fair value		$ 28
orarily impaired securities, 12 months or longer, unrealized losses		$ 20
Temporarily impaired securities, 12 months or longer, no. Of issuances		1
porarily impaired securities, total fair value	$ 1,516	$ 28
porarily impaired securities, total unrealized losses	$ 17	$ 20
Temporarily impaired securities, total no. Of issuances	5	1
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 5,015
Temporarily impaired securities, less than 12 months, unrealized losses	$ 17
Temporarily impaired securities, less than 12 months, no. Of issuances	1
Temporarily impaired securities, 12 months or longer, fair value		$ 29,138
orarily impaired securities, 12 months or longer, unrealized losses		$ 862
Temporarily impaired securities, 12 months or longer, no. Of issuances		2
porarily impaired securities, total fair value	$ 5,015	$ 29,138
porarily impaired securities, total unrealized losses	$ 17	$ 862
Temporarily impaired securities, total no. Of issuances	1	2
Mutual Funds [Member]
Temporarily impaired securities, less than 12 months, fair value
Temporarily impaired securities, less than 12 months, unrealized losses
Temporarily impaired securities, less than 12 months, no. Of issuances
Temporarily impaired securities, 12 months or longer, fair value	$ 6,230	$ 6,230
orarily impaired securities, 12 months or longer, unrealized losses	$ 270	$ 270
Temporarily impaired securities, 12 months or longer, no. Of issuances	1	1
porarily impaired securities, total fair value	$ 6,230	$ 6,230
porarily impaired securities, total unrealized losses	$ 270	$ 270
Temporarily impaired securities, total no. Of issuances	1	1