Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 825,104	$ 915,628
Tier I (leverage) capital, capital level, percent	0.0947	0.1056
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 435,792	$ 433,667
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 389,312	$ 481,961
Tier I (leverage) capital, excess, percent	0.0447	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 825,104	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1222	0.1379
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 438,878	$ 431,734
Common Equity Tier I risk-based capital, requirement to be well-capitalized, percent	0.065%	0.065%
Common Equity Tier I risk-based capital, excess, amount	$ 386,226	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0572%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 825,104	$ 915,628
Tier I risk-based capital, capital level, percent	0.1222	0.1379
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 540,157	$ 531,365
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 284,947	$ 384,263
Tier I risk-based capital, excess, percent	0.0422	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 864,999	$ 954,457
Total risk-based capital, capital level, percent	0.1281	0.1437
Total risk-based capital, requirement to be well-capitalized, amount	$ 675,196	$ 664,206
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 189,803	$ 290,251
Total risk-based capital, excess, percent	0.0281	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 737,732	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0847	0.0861
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 435,748	$ 433,607
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 301,984	$ 313,273
Tier I (leverage) capital, excess, percent	0.0347	0.0361
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 691,754	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1025	0.1052
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 438,770	$ 431,635
Common Equity Tier I risk-based capital, requirement to be well-capitalized, percent	0.065%	0.065%
Common Equity Tier I risk-based capital, excess, amount	$ 252,984	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0375%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 737,732	$ 746,880
Tier I risk-based capital, capital level, percent	0.1093	0.1125
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 540,024	$ 531,243
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 197,708	$ 215,637
Tier I risk-based capital, excess, percent	0.0293	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 967,627	$ 975,709
Total risk-based capital, capital level, percent	0.1433	0.1469
Total risk-based capital, requirement to be well-capitalized, amount	$ 675,030	$ 664,054
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 292,597	$ 311,655
Total risk-based capital, excess, percent	0.0433	0.0469