Note 14 - Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Savings Bank [Member]
Tier I (leverage) capital:
Tier i (leverage) capital, capital level, amount	$ 616,017	$ 607,033
Tier i (leverage) capital, capital level, percent	9.98%	10.12%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 308,535	$ 299,848
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 307,482	$ 307,185
Tier i (leverage) capital, excess, percent	4.98%	5.12%
Common Equity Tier I risk-based capital:
Common equity tier i risk-based capital, capital level, amount	$ 616,017	$ 607,033
Common equity tier i risk-based capital, capital level, percent	13.80%	14.12%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 290,046	$ 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	$ 325,971	$ 327,590
Common equity tier i risk-based capital, excess, percent	7.30%	7.62%
Tier I risk-based capital:
Tier 1 risk-based capital, capital level, amount	$ 616,017	$ 607,033
Tier 1 risk-based capital, capital level, percent	13.80%	14.12%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 356,980	$ 343,930
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 259,037	$ 263,103
Tier 1 risk-based capital, excess, percent	5.80%	6.12%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 638,228	$ 629,262
Total risk-based capital, capital level, percent	14.30%	14.64%
Total risk-based capital, requirement to be well capitalized, amount	$ 446,225	$ 429,913
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 192,003	$ 199,349
Total risk-based capital, excess, percent	4.30%	4.64%
Holding Company [Member]
Tier I (leverage) capital:
Tier i (leverage) capital, capital level, amount	$ 550,055	$ 539,228
Tier i (leverage) capital, capital level, percent	8.92%	9.00%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 308,284	$ 299,654
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 241,771	$ 239,574
Tier i (leverage) capital, excess, percent	3.92%	4.00%
Common Equity Tier I risk-based capital:
Common equity tier i risk-based capital, capital level, amount	$ 516,706	$ 506,432
Common equity tier i risk-based capital, capital level, percent	11.59%	11.79%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 289,733	$ 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	$ 226,973	$ 227,311
Common equity tier i risk-based capital, excess, percent	5.09%	5.29%
Tier I risk-based capital:
Tier 1 risk-based capital, capital level, amount	$ 550,055	$ 539,228
Tier 1 risk-based capital, capital level, percent	12.34%	12.56%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 356,594	$ 343,534
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 193,461	$ 195,694
Tier 1 risk-based capital, excess, percent	4.34%	4.56%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 647,266	$ 636,457
Total risk-based capital, capital level, percent	14.52%	14.82%
Total risk-based capital, requirement to be well capitalized, amount	$ 445,743	$ 429,417
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 201,523	$ 207,040
Total risk-based capital, excess, percent	4.52%	4.82%