Note 4 - Investment Securities - Temporarily Impaired Securities (Details) $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Temporarily impaired securities, less than 12 months, fair value	$ 1,017,655	$ 1,410,387
Temporarily impaired securities, less than 12 months, unrealized losses	$ 13,814	$ 14,253
Temporarily impaired securities, less than 12 months, no. Of issuances	30	47
Temporarily impaired securities, 12 months or longer, fair value	$ 35,458	$ 49,400
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 1,154	$ 1,670
Temporarily impaired securities, 12 months or longer, no. Of issuances	7	7
Temporarily impaired securities, total fair value	$ 1,053,113	$ 1,459,787
Temporarily impaired securities, total unrealized losses	$ 14,968	$ 15,923
Temporarily impaired securities, total no. Of issuances	37	54
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 299,088	$ 224,289
Temporarily impaired securities, less than 12 months, unrealized losses	$ 857	$ 395
Temporarily impaired securities, less than 12 months, no. Of issuances	6	5
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
Temporarily impaired securities, total fair value	$ 299,088	$ 224,289
Temporarily impaired securities, total unrealized losses	$ 857	$ 395
Temporarily impaired securities, total no. Of issuances	6	5
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 390,331	$ 148,160
Temporarily impaired securities, less than 12 months, unrealized losses	$ 9,669	$ 1,840
Temporarily impaired securities, less than 12 months, no. Of issuances	8	3
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
Temporarily impaired securities, total fair value	$ 390,331	$ 148,160
Temporarily impaired securities, total unrealized losses	$ 9,669	$ 1,840
Temporarily impaired securities, total no. Of issuances	8	3
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 328,236	$ 1,025,342
Temporarily impaired securities, less than 12 months, unrealized losses	$ 3,288	$ 11,398
Temporarily impaired securities, less than 12 months, no. Of issuances	16	35
Temporarily impaired securities, 12 months or longer, fair value	$ 62	$ 6
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 2	$ 1
Temporarily impaired securities, 12 months or longer, no. Of issuances	3	2
Temporarily impaired securities, total fair value	$ 328,298	$ 1,025,348
Temporarily impaired securities, total unrealized losses	$ 3,290	$ 11,399
Temporarily impaired securities, total no. Of issuances	19	37
Collateralized Mortgage Obligations [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	$ 28	$ 36
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 20	$ 27
Temporarily impaired securities, 12 months or longer, no. Of issuances	1	1
Temporarily impaired securities, total fair value	$ 28	$ 36
Temporarily impaired securities, total unrealized losses	$ 20	$ 27
Temporarily impaired securities, total no. Of issuances	1	1
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		$ 9,950
Temporarily impaired securities, less than 12 months, unrealized losses		$ 50
Temporarily impaired securities, less than 12 months, no. Of issuances		1
Temporarily impaired securities, 12 months or longer, fair value	$ 29,138	$ 43,525
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 862	$ 1,475
Temporarily impaired securities, 12 months or longer, no. Of issuances	2	3
Temporarily impaired securities, total fair value	$ 29,138	$ 53,475
Temporarily impaired securities, total unrealized losses	$ 862	$ 1,525
Temporarily impaired securities, total no. Of issuances	2	4
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	$ 5,833
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 270	$ 167
Temporarily impaired securities, 12 months or longer, no. Of issuances	1	1
Temporarily impaired securities, total fair value	$ 6,230	$ 5,833
Temporarily impaired securities, total unrealized losses	$ 270	$ 167
Temporarily impaired securities, total no. Of issuances	1	1
Preferred Stock of Government Sponsored Entities [Member]
Temporarily impaired securities, less than 12 months, fair value		$ 2,488
Temporarily impaired securities, less than 12 months, unrealized losses		$ 228
Temporarily impaired securities, less than 12 months, no. Of issuances		2
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
Temporarily impaired securities, total fair value		$ 2,488
Temporarily impaired securities, total unrealized losses		$ 228
Temporarily impaired securities, total no. Of issuances		2
Other Equity Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		$ 158
Temporarily impaired securities, less than 12 months, unrealized losses		$ 342
Temporarily impaired securities, less than 12 months, no. Of issuances		1
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, 12 months or longer, no. Of issuances
Temporarily impaired securities, total fair value		$ 158
Temporarily impaired securities, total unrealized losses		$ 342
Temporarily impaired securities, total no. Of issuances		1