Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 914,191	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1066	0.1056
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 428,900	$ 433,667
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 485,291	$ 481,961
Tier I (leverage) capital, excess, percent	0.0566	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 914,191	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1374	0.1379
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 432,376	$ 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	$ 481,815	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0724%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 914,191	$ 915,628
Tier I risk-based capital, capital level, percent	0.1374	0.1379
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 532,155	$ 531,365
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 382,036	$ 384,263
Tier I risk-based capital, excess, percent	0.0574	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 952,221	$ 954,457
Total risk-based capital, capital level, percent	0.1431	0.1437
Total risk-based capital, requirement to be well-capitalized, amount	$ 665,194	$ 664,206
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 287,027	$ 290,251
Total risk-based capital, excess, percent	0.0431	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 734,754	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0854	0.0861
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 429,936	$ 433,607
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 304,818	$ 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	$ 688,820	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1036	0.1052
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 432,264	$ 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	$ 256,556	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0386%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 734,754	$ 746,880
Tier I risk-based capital, capital level, percent	0.1105	0.1125
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 532,018	$ 531,243
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 202,736	$ 215,637
Tier I risk-based capital, excess, percent	0.0305	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 962,784	$ 975,709
Total risk-based capital, capital level, percent	0.1448	0.1469
Total risk-based capital, requirement to be well-capitalized, amount	$ 665,022	$ 664,054
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 297,762	$ 311,655
Total risk-based capital, excess, percent	0.0448	0.0469