Note 15 - Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Savings Bank [Member]
Tier i (leverage) capital, capital level, amount	$ 663,467	$ 660,782
Tier i (leverage) capital, capital level, percent	9.64%	9.85%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 344,216	$ 335,512
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 319,251	$ 325,270
Tier i (leverage) capital, excess, percent	4.64%	4.85%
Common equity tier i risk-based capital, capital level, amount	$ 663,467	$ 660,782
Common equity tier i risk-based capital, capital level, percent	13.08%	13.28%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 329,701	$ 323,386
Common equity tier i risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common equity tier i risk-based capital, excess, amount	$ 333,766	$ 337,396
Common equity tier i risk-based capital, excess, percent	6.58%	6.78%
Tier 1 risk-based capital, capital level, amount	$ 663,467	$ 660,782
Tier 1 risk-based capital, capital level, percent	13.08%	13.28%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 405,786	$ 398,014
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 257,681	$ 262,768
Tier 1 risk-based capital, excess, percent	5.08%	5.28%
Total risk-based capital, capital level, amount	$ 684,482	$ 681,727
Total risk-based capital, capital level, percent	13.49%	13.70%
Total risk-based capital, requirement to be well capitalized, amount	$ 507,232	$ 497,517
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 177,250	$ 184,210
Total risk-based capital, excess, percent	3.49%	3.70%
Holding Company [Member]
Tier i (leverage) capital, capital level, amount	$ 594,196	$ 586,582
Tier i (leverage) capital, capital level, percent	8.63%	8.74%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 344,147	$ 335,616
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 250,049	$ 250,966
Tier i (leverage) capital, excess, percent	3.63%	3.74%
Common equity tier i risk-based capital, capital level, amount	$ 552,793	$ 546,230
Common equity tier i risk-based capital, capital level, percent	10.90%	10.98%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 329,661	$ 323,382
Common equity tier i risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common equity tier i risk-based capital, excess, amount	$ 223,132	$ 222,848
Common equity tier i risk-based capital, excess, percent	4.40%	4.48%
Tier 1 risk-based capital, capital level, amount	$ 594,196	$ 586,582
Tier 1 risk-based capital, capital level, percent	11.72%	11.79%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 405,737	$ 398,008
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 188,459	$ 188,574
Tier 1 risk-based capital, excess, percent	3.72%	3.79%
Total risk-based capital, capital level, amount	$ 690,211	$ 682,527
Total risk-based capital, capital level, percent	13.61%	13.72%
Total risk-based capital, requirement to be well capitalized, amount	$ 507,171	$ 497,511
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 183,040	$ 185,016
Total risk-based capital, excess, percent	3.61%	3.72%