Note 6 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Temporarily impaired securities, less than 12 months, fair value	$ 1,130,844	$ 1,017,655
Temporarily impaired securities, less than 12 months, unrealized losses	11,552	13,814
Temporarily impaired securities, 12 months or longer, fair value	35,959	35,458
Temporarily impaired securities, 12 months or longer, unrealized losses	602	1,154
Temporarily impaired securities, total fair value	1,166,803	1,053,113
Temporarily impaired securities, total unrealized losses	12,154	14,968
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	419,470	299,088
Temporarily impaired securities, less than 12 months, unrealized losses	411	857
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	419,470	299,088
Temporarily impaired securities, total unrealized losses	411	857
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	391,986	390,331
Temporarily impaired securities, less than 12 months, unrealized losses	8,014	9,669
Temporarily impaired securities, 12 months or longer, fair value
Temporarily impaired securities, 12 months or longer, unrealized losses
Temporarily impaired securities, total fair value	391,986	390,331
Temporarily impaired securities, total unrealized losses	8,014	9,669
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	314,395	328,236
Temporarily impaired securities, less than 12 months, unrealized losses	3,120	3,288
Temporarily impaired securities, 12 months or longer, fair value	58	62
Temporarily impaired securities, 12 months or longer, unrealized losses	3	2
Temporarily impaired securities, total fair value	314,453	328,298
Temporarily impaired securities, total unrealized losses	3,123	3,290
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	4,993
Temporarily impaired securities, less than 12 months, unrealized losses	7
Temporarily impaired securities, 12 months or longer, fair value	29,671	29,138
Temporarily impaired securities, 12 months or longer, unrealized losses	329	862
Temporarily impaired securities, total fair value	34,664	29,138
Temporarily impaired securities, total unrealized losses	336	862
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, 12 months or longer, fair value		28
Temporarily impaired securities, 12 months or longer, unrealized losses		20
Temporarily impaired securities, total fair value		28
Temporarily impaired securities, total unrealized losses		20
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	6,230	6,230
Temporarily impaired securities, 12 months or longer, unrealized losses	270	270
Temporarily impaired securities, total fair value	6,230	6,230
Temporarily impaired securities, total unrealized losses	$ 270	$ 270