Note 3 - Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost	$ 406,033	$ 462,096
Gross Unrealized Gains	6,493	5,137
Gross Unrealized Losses	(386)	(1,273)
Estimated Fair Value	412,140	465,960
Amortized Cost	1,333,815	1,181,484
Gross Unrealized Gains	1,939	1,279
Gross Unrealized Losses	(18,334)	(25,409)
Estimated Fair Value	1,317,420	1,157,354
US Government Agencies Debt Securities [Member]
Amortized Cost	66,928	76,875
Gross Unrealized Gains	40	107
Gross Unrealized Losses	(140)	(182)
Estimated Fair Value	66,828	76,800
Amortized Cost	210,004	140,005
Gross Unrealized Gains	273	67
Gross Unrealized Losses	(2,057)	(2,301)
Estimated Fair Value	208,220	137,771
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Amortized Cost	16,972	19,773
Gross Unrealized Gains	56	63
Gross Unrealized Losses	(191)	(249)
Estimated Fair Value	16,837	19,587
Amortized Cost	973,111	885,783
Gross Unrealized Gains	120	178
Gross Unrealized Losses	(13,533)	(17,637)
Estimated Fair Value	959,698	868,324
US States and Political Subdivisions Debt Securities [Member]
Amortized Cost	320,116	362,532
Gross Unrealized Gains	6,397	4,967
Gross Unrealized Losses	(55)	(842)
Estimated Fair Value	326,458	366,657
Amortized Cost	87,405	108,374
Gross Unrealized Gains	161	38
Gross Unrealized Losses	(2,744)	(5,469)
Estimated Fair Value	84,822	102,943
Other Securities [Member]
Amortized Cost	2,017	2,916
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value	2,017	2,916
Amortized Cost	63,295	47,022
Gross Unrealized Gains	1,385	996
Gross Unrealized Losses		(2)
Estimated Fair Value	64,680	48,016
US Treasury Securities [Member]
Amortized Cost		300
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value		$ 300