Note 16 - Disclosures About Fair Value of Financial Instruments - Fair Value of Financial Assets Measure on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities	$ 1,157,354	$ 821,407
Assets held in trading accounts	41	4,422
Fair Value, Measurements, Recurring [Member]
Assets held in trading accounts	41	4,422
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets held in trading accounts		2,409
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets held in trading accounts	41	2,013
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets held in trading accounts
US Treasury Securities [Member]
Available-for-sale securities	300	3,994
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	300	3,994
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities	300	3,994
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
US Government Agencies Debt Securities [Member]
Available-for-sale securities	137,771	120,237
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	137,771	120,237
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	137,771	120,237
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-sale securities	868,324	647,426
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	868,324	647,426
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	868,324	647,426
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	102,943	9,874
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	102,943	9,874
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	102,943	9,874
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
Other Securities [Member]
Available-for-sale securities	48,016	39,876
Other Securities [Member] | Fair Value, Measurements, Recurring [Member]
Other securities	48,016	39,876
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Other securities
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Other securities	39,876	39,876
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Other securities