Note 14 - Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Savings Bank [Member]
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 313,185	$ 307,185
Tier i (leverage) capital, excess, percent	5.04%	5.12%
Common Equity Tier I risk-based capital:
Common equity tier i risk-based capital, capital level, amount	$ 624,074	$ 607,033
Common equity tier i risk-based capital, capital level, percent	13.58%	14.12%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 298,648	$ 279,443
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	$ 325,426	$ 327,590
Common equity tier i risk-based capital, excess, percent	7.08%	7.62%
Tier 1 risk-based capital:
Tier 1 risk-based capital, capital level, amount	$ 624,074	$ 607,033
Tier 1 risk-based capital, capital level, percent	13.58%	14.12%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 367,567	$ 343,930
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 256,507	$ 263,103
Tier 1 risk-based capital, excess, percent	5.58%	6.12%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 646,231	$ 629,262
Total risk-based capital, capital level, percent	14.07%	14.64%
Total risk-based capital, requirement to be well capitalized, amount	$ 459,459	$ 429,913
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 186,772	$ 199,349
Total risk-based capital, excess, percent	4.07%	4.64%
Tier i (leverage) capital, capital level, amount	$ 624,074	$ 607,033
Tier i (leverage) capital, capital level, percent	10.04%	10.12%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 310,889	$ 299,848
Holding Company [Member]
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 248,202	$ 239,574
Tier i (leverage) capital, excess, percent	4.00%	4.00%
Common Equity Tier I risk-based capital:
Common equity tier i risk-based capital, capital level, amount	$ 524,830	$ 506,432
Common equity tier i risk-based capital, capital level, percent	11.43%	11.79%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 298,350	$ 279,121
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	$ 226,480	$ 227,311
Common equity tier i risk-based capital, excess, percent	4.93%	5.29%
Tier 1 risk-based capital:
Tier 1 risk-based capital, capital level, amount	$ 558,756	$ 539,228
Tier 1 risk-based capital, capital level, percent	12.17%	12.56%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 367,199	$ 343,534
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 191,557	$ 195,694
Tier 1 risk-based capital, excess, percent	4.17%	4.56%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 655,913	$ 636,457
Total risk-based capital, capital level, percent	14.29%	14.82%
Total risk-based capital, requirement to be well capitalized, amount	$ 458,999	$ 429,417
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 196,914	$ 207,040
Total risk-based capital, excess, percent	4.29%	4.82%
Tier i (leverage) capital, capital level, amount	$ 558,756	$ 539,228
Tier i (leverage) capital, capital level, percent	9.00%	9.00%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 310,554	$ 299,654