Note 4 - Investment Securities - Temporarily Impaired Securities (Details) $ in Thousands	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Temporarily impaired securities, less than 12 months, fair value	$ 176,667	$ 556,415
Temporarily impaired securities, less than 12 months, unrealized losses	$ 630	$ 3,343
Temporarily impaired securities, less than 12 months, no. Of issuances	14	63
Temporarily impaired securities, 12 months or longer, fair value	$ 899,523	$ 624,880
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 26,370	$ 13,341
Temporarily impaired securities, 12 months or longer, no. Of issuances	90	24
Temporarily impaired securities, total fair value	$ 1,076,190	$ 1,181,295
Temporarily impaired securities, total unrealized losses	$ 27,000	$ 16,684
Temporarily impaired securities, total no. Of issuances	104	87
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 124,751	$ 199,823
Temporarily impaired securities, less than 12 months, unrealized losses	$ 51	$ 62
Temporarily impaired securities, less than 12 months, no. Of issuances	4	4
Temporarily impaired securities, 12 months or longer, fair value		$ 49,697
Temporarily impaired securities, 12 months or longer, unrealized losses		$ 295
Temporarily impaired securities, 12 months or longer, no. Of issuances		2
Temporarily impaired securities, total fair value	$ 124,751	$ 249,520
Temporarily impaired securities, total unrealized losses	$ 51	$ 357
Temporarily impaired securities, total no. Of issuances	4	6
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 3,388	$ 5,711
Temporarily impaired securities, less than 12 months, unrealized losses	$ 77	$ 70
Temporarily impaired securities, less than 12 months, no. Of issuances	2	3
Temporarily impaired securities, 12 months or longer, fair value	$ 2,483
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 118
Temporarily impaired securities, 12 months or longer, no. Of issuances	2
Temporarily impaired securities, total fair value	$ 5,871	$ 5,711
Temporarily impaired securities, total unrealized losses	$ 195	$ 70
Temporarily impaired securities, total no. Of issuances	4	3
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, less than 12 months, no. Of issuances
Temporarily impaired securities, 12 months or longer, fair value	$ 388,362	$ 390,336
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 11,637	$ 9,664
Temporarily impaired securities, 12 months or longer, no. Of issuances	8	8
Temporarily impaired securities, total fair value	$ 388,362	$ 390,336
Temporarily impaired securities, total unrealized losses	$ 11,637	$ 9,664
Temporarily impaired securities, total no. Of issuances	8	8
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	$ 48,528	$ 342,436
Temporarily impaired securities, less than 12 months, unrealized losses	$ 502	$ 3,147
Temporarily impaired securities, less than 12 months, no. Of issuances	8	48
Temporarily impaired securities, 12 months or longer, fair value	$ 507,701	$ 178,617
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 14,587	$ 3,112
Temporarily impaired securities, 12 months or longer, no. Of issuances	74	13
Temporarily impaired securities, total fair value	$ 556,229	$ 521,053
Temporarily impaired securities, total unrealized losses	$ 15,089	$ 6,259
Temporarily impaired securities, total no. Of issuances	82	61
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
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		$ 1,914
Temporarily 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	$ 977
Temporarily impaired securities, 12 months or longer, unrealized losses	$ 28
Temporarily impaired securities, 12 months or longer, no. Of issuances	6
Temporarily impaired securities, total fair value	$ 977	$ 1,516
Temporarily impaired securities, total unrealized losses	$ 28	$ 17
Temporarily impaired securities, total no. Of issuances	6	5
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
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		$ 5,015
Temporarily impaired securities, total unrealized losses		$ 17
Temporarily impaired securities, total no. Of issuances		1
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
Temporarily impaired securities, 12 months or longer, unrealized losses		$ 270
Temporarily impaired securities, 12 months or longer, no. Of issuances		1
Temporarily impaired securities, total fair value		$ 6,230
Temporarily impaired securities, total unrealized losses		$ 270
Temporarily impaired securities, total no. Of issuances		1