Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 733,010	$ 680,749
Tier I (leverage) capital, capital level, percent	9.27%	9.65%
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 395,510	$ 352,581
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier I (leverage) capital, excess, amount	$ 337,500	$ 328,168
Tier I (leverage) capital, excess, percent	4.27%	4.65%
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 733,010	$ 680,749
Common Equity Tier I risk-based capital, capital level, percent	11.65%	13.02%
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 408,929	$ 339,944
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	$ 324,081	$ 340,805
Common Equity Tier I risk-based capital, excess, percent	5.15%	6.52%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 733,010	$ 680,749
Tier I risk-based capital, capital level, percent	11.65%	13.02%
Tier I risk-based capital, requirement to be well capitalized, amount	$ 503,297	$ 418,393
Tier I risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier I risk-based capital, excess, amount	$ 229,713	$ 262,356
Tier I risk-based capital, excess, percent	3.65%	5.02%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 773,807	$ 702,500
Total risk-based capital, capital level, percent	12.30%	13.43%
Total risk-based capital, requirement to be well capitalized, amount	$ 629,121	$ 522,991
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 144,686	$ 179,509
Total risk-based capital, excess, percent	2.30%	3.43%
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 662,987	$ 615,500
Tier I (leverage) capital, capital level, percent	8.38%	8.73%
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 395,439	$ 352,581
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier I (leverage) capital, excess, amount	$ 267,548	$ 262,919
Tier I (leverage) capital, excess, percent	3.38%	3.73%
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 621,247	$ 572,651
Common Equity Tier I risk-based capital, capital level, percent	9.88%	10.95%
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 408,694	$ 339,929
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	$ 212,553	$ 232,722
Common Equity Tier I risk-based capital, excess, percent	3.38%	4.45%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 662,987	$ 615,500
Tier I risk-based capital, capital level, percent	10.54%	11.77%
Tier I risk-based capital, requirement to be well capitalized, amount	$ 503,008	$ 418,374
Tier I risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier I risk-based capital, excess, amount	$ 159,979	$ 197,126
Tier I risk-based capital, excess, percent	2.54%	3.77%
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 794,034	$ 712,251
Total risk-based capital, capital level, percent	12.63%	13.62%
Total risk-based capital, requirement to be well capitalized, amount	$ 628,760	$ 522,967
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 165,274	$ 189,284
Total risk-based capital, excess, percent	2.63%	3.62%