Note 6 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Temporarily impaired securities, less than 12 months, fair value	$ 1,265,672	$ 1,017,655
Temporarily impaired securities, less than 12 months, unrealized losses	8,370	13,814
Temporarily impaired securities, 12 months or longer, fair value	36,212	35,458
Temporarily impaired securities, 12 months or longer, unrealized losses	347	1,154
Temporarily impaired securities, total fair value	1,301,884	1,053,113
Temporarily impaired securities, total unrealized losses	8,717	14,968
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	569,044	299,088
Temporarily impaired securities, less than 12 months, unrealized losses	524	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	569,044	299,088
Temporarily impaired securities, total unrealized losses	524	857
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	394,631	390,331
Temporarily impaired securities, less than 12 months, unrealized losses	5,369	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	394,631	390,331
Temporarily impaired securities, total unrealized losses	5,369	9,669
Collateralized Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	301,997	328,236
Temporarily impaired securities, less than 12 months, unrealized losses	2,477	3,288
Temporarily impaired securities, 12 months or longer, fair value	56	62
Temporarily impaired securities, 12 months or longer, unrealized losses	3	2
Temporarily impaired securities, total fair value	302,053	328,298
Temporarily impaired securities, total unrealized losses	2,480	3,290
Corporate Debt Securities [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	29,891	29,138
Temporarily impaired securities, 12 months or longer, unrealized losses	109	862
Temporarily impaired securities, total fair value	29,891	29,138
Temporarily impaired securities, total unrealized losses	109	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	6,265	6,230
Temporarily impaired securities, 12 months or longer, unrealized losses	235	270
Temporarily impaired securities, total fair value	6,265	6,230
Temporarily impaired securities, total unrealized losses	$ 235	$ 270