Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 918,167	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1061	0.1056
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 432,771	$ 433,667
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 485,396	$ 481,961
Tier I (leverage) capital, excess, percent	0.0561	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 918,167	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1349	0.1379
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 442,255	$ 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	$ 475,912	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0699%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 918,167	$ 915,628
Tier I risk-based capital, capital level, percent	0.1349	0.1379
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 544,314	$ 531,365
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 373,853	$ 384,263
Tier I risk-based capital, excess, percent	0.0549	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 956,955	$ 954,457
Total risk-based capital, capital level, percent	0.1406	0.1437
Total risk-based capital, requirement to be well-capitalized, amount	$ 680,393	$ 664,206
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 276,562	$ 290,251
Total risk-based capital, excess, percent	0.0406	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 739,364	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0854	0.0861
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 432,714	$ 433,607
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 306,650	$ 313,273
Tier I (leverage) capital, excess, percent	0.0354	0.0361
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 692,914	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1019	0.1052
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 442,155	$ 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	$ 250,759	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0369%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 739,364	$ 746,880
Tier I risk-based capital, capital level, percent	0.1087	0.1125
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 544,191	$ 531,243
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 195,173	$ 215,637
Tier I risk-based capital, excess, percent	0.0287	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 968,152	$ 975,709
Total risk-based capital, capital level, percent	0.1423	0.1469
Total risk-based capital, requirement to be well-capitalized, amount	$ 680,239	$ 664,054
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 287,913	$ 311,655
Total risk-based capital, excess, percent	0.0423	0.0469