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Note 3 - Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Held-to-Maturity
Amortized Cost	$ 368,058	$ 462,096
Gross Unrealized Gains	5,977	5,137
Gross Unrealized Losses	(737)	(1,273)
Estimated Fair Value	373,298	465,960
Available-for-Sale
Amortized Cost	1,612,324	1,181,484
Gross Unrealized Gains	2,100	1,279
Gross Unrealized Losses	(24,907)	(25,409)
Estimated Fair Value	1,589,517	1,157,354
US Government Agencies Debt Securities [Member]
Held-to-Maturity
Amortized Cost	46,945	76,875
Gross Unrealized Gains	7	107
Gross Unrealized Losses	(228)	(182)
Estimated Fair Value	46,724	76,800
Available-for-Sale
Amortized Cost	141,559	140,005
Gross Unrealized Gains	116	67
Gross Unrealized Losses	(1,951)	(2,301)
Estimated Fair Value	139,724	137,771
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Held-to-Maturity
Amortized Cost	16,132	19,773
Gross Unrealized Gains	8	63
Gross Unrealized Losses	(287)	(249)
Estimated Fair Value	15,853	19,587
Available-for-Sale
Amortized Cost	1,208,017	885,783
Gross Unrealized Gains	246	178
Gross Unrealized Losses	(20,946)	(17,637)
Estimated Fair Value	1,187,317	868,324
US States and Political Subdivisions Debt Securities [Member]
Held-to-Maturity
Amortized Cost	301,491	362,532
Gross Unrealized Gains	5,962	4,967
Gross Unrealized Losses	(222)	(842)
Estimated Fair Value	307,231	366,657
Available-for-Sale
Amortized Cost	144,642	108,374
Gross Unrealized Gains	532	38
Gross Unrealized Losses	(2,009)	(5,469)
Estimated Fair Value	143,165	102,943
Other Securities [Member]
Held-to-Maturity
Amortized Cost	3,490	2,916
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value	3,490	2,916
Available-for-Sale
Amortized Cost	118,106	47,022
Gross Unrealized Gains	1,206	996
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	119,311	48,016
US Treasury Securities [Member]
Available-for-Sale
Amortized Cost		300
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value		$ 300

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