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	$ 915,547	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1058	0.1056
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 432,820	$ 433,667
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 482,727	$ 481,961
Tier I (leverage) capital, excess, percent	0.0558	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 915,547	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1345	0.1379
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 442,391	$ 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	$ 473,156	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0695%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 915,547	$ 915,628
Tier I risk-based capital, capital level, percent	0.1345	0.1379
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 544,482	$ 531,365
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 371,065	$ 384,263
Tier I risk-based capital, excess, percent	0.0545	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 954,335	$ 954,457
Total risk-based capital, capital level, percent	0.1402	0.1437
Total risk-based capital, requirement to be well-capitalized, amount	$ 680,602	$ 664,206
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 273,733	$ 290,251
Total risk-based capital, excess, percent	0.0402	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 736,744	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0851	0.0861
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 432,763	$ 433,607
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 303,981	$ 313,273
Tier I (leverage) capital, excess, percent	0.0351	0.0361
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 690,294	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1014	0.1052
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 442,291	$ 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	$ 248,003	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0364%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 736,744	$ 746,880
Tier I risk-based capital, capital level, percent	0.1083	0.1125
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 544,358	$ 531,243
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 192,386	$ 215,637
Tier I risk-based capital, excess, percent	0.0283	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 965,532	$ 975,709
Total risk-based capital, capital level, percent	0.1419	0.1469
Total risk-based capital, requirement to be well-capitalized, amount	$ 680,448	$ 664,054
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 285,084	$ 311,655
Total risk-based capital, excess, percent	0.0419	0.0469