Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 829,971	$ 825,104
Tier I (leverage) capital, capital level, percent	0.0940	0.0947
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 441,496	$ 435,792
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 388,475	$ 389,312
Tier I (leverage) capital, excess, percent	0.0440	0.0447
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 829,971	$ 825,104
Common Equity Tier I risk-based capital, capital level, percent	0.1246	0.1222
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 433,032	$ 438,878
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	$ 396,939	$ 386,226
Common Equity Tier I risk-based capital, excess, percent	0.0596%	0.0572%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 829,971	$ 825,104
Tier I risk-based capital, capital level, percent	0.1246	0.1222
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 532,962	$ 540,157
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 297,009	$ 284,947
Tier I risk-based capital, excess, percent	0.0446	0.0422
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 871,575	$ 864,999
Total risk-based capital, capital level, percent	0.1308	0.1281
Total risk-based capital, requirement to be well-capitalized, amount	$ 666,203	$ 675,196
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 205,372	$ 189,803
Total risk-based capital, excess, percent	0.0308	0.0281
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 734,192	$ 737,732
Tier I (leverage) capital, capital level, percent	0.0832	0.0847
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 441,455	$ 435,748
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 292,737	$ 301,984
Tier I (leverage) capital, excess, percent	0.0332	0.0347
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 687,458	$ 691,754
Common Equity Tier I risk-based capital, capital level, percent	0.1032	0.1025
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 433,192	$ 438,770
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	$ 254,266	$ 252,984
Common Equity Tier I risk-based capital, excess, percent	0.0382%	0.0375%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 734,192	$ 737,732
Tier I risk-based capital, capital level, percent	0.1102	0.1093
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 533,160	$ 540,024
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 201,032	$ 197,708
Tier I risk-based capital, excess, percent	0.0302	0.0293
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 965,796	$ 967,627
Total risk-based capital, capital level, percent	0.1449	0.1433
Total risk-based capital, requirement to be well-capitalized, amount	$ 666,450	$ 675,030
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 299,346	$ 292,597
Total risk-based capital, excess, percent	0.0449	0.0433