Note 3 - Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Amortized Cost	$ 462,096	$ 705,373
Gross Unrealized Gains	5,137	9,806
Gross Unrealized Losses	(1,273)	(1,808)
Held-to-maturity securities	465,960	713,371
Amortized Cost	1,181,484	824,992
Gross Unrealized Gains	1,279	1,587
Gross Unrealized Losses	(25,409)	(5,172)
Available-for-sale securities	1,157,354	821,407
US Government Agencies Debt Securities [Member]
Amortized Cost	76,875	237,139
Gross Unrealized Gains	107	582
Gross Unrealized Losses	(182)	(1,395)
Held-to-maturity securities	76,800	236,326
Amortized Cost	140,005	121,017
Gross Unrealized Gains	67	118
Gross Unrealized Losses	(2,301)	(898)
Available-for-sale securities	137,771	120,237
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Amortized Cost	19,773	24,774
Gross Unrealized Gains	63	86
Gross Unrealized Losses	(249)	(290)
Held-to-maturity securities	19,587	24,570
Amortized Cost	885,783	650,619
Gross Unrealized Gains	178	937
Gross Unrealized Losses	(17,637)	(4,130)
Available-for-sale securities	868,324	647,426
US States and Political Subdivisions Debt Securities [Member]
Amortized Cost	362,532	440,676
Gross Unrealized Gains	4,967	9,138
Gross Unrealized Losses	(842)	(123)
Held-to-maturity securities	366,657	449,691
Amortized Cost	108,374	9,762
Gross Unrealized Gains	38	112
Gross Unrealized Losses	(5,469)
Available-for-sale securities	102,943	9,874
Other Securities [Member]
Amortized Cost	2,916	2,784
Gross Unrealized Gains
Gross Unrealized Losses
Held-to-maturity securities	2,916	2,784
Amortized Cost	47,022	39,594
Gross Unrealized Gains	996	420
Gross Unrealized Losses	(2)	(138)
Available-for-sale securities	48,016	39,876
US Treasury Securities [Member]
Amortized Cost	300	4,000
Gross Unrealized Gains
Gross Unrealized Losses		(6)
Available-for-sale securities	$ 300	$ 3,994