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	$ 915,247	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1067	0.1056
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 428,869	$ 433,667
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 486,378	$ 481,961
Tier I (leverage) capital, excess, percent	0.0567	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 915,247	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1376	0.1379
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 432,313	$ 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	$ 482,934	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0726%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 915,247	$ 915,628
Tier I risk-based capital, capital level, percent	0.1376	0.1379
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 532,077	$ 531,365
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 383,170	$ 384,263
Tier I risk-based capital, excess, percent	0.0576	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 953,277	$ 954,457
Total risk-based capital, capital level, percent	0.1433	0.1437
Total risk-based capital, requirement to be well-capitalized, amount	$ 665,097	$ 664,206
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 288,180	$ 290,251
Total risk-based capital, excess, percent	0.0433	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 735,810	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0856	0.0861
Tier I (leverage) capital, requirement to be well-capitalized, amount	$ 429,905	$ 433,607
Tier I (leverage) capital, requirement to be well-capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 305,905	$ 313,273
Tier I (leverage) capital, excess, percent	0.0356	0.0361
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 689,876	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1038	0.1052
Common Equity Tier I risk-based capital, requirement to be well-capitalized, amount	$ 432,201	$ 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	$ 257,675	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0388%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 735,810	$ 746,880
Tier I risk-based capital, capital level, percent	0.1107	0.1125
Tier I risk-based capital, requirement to be well-capitalized, amount	$ 531,940	$ 531,243
Tier I risk-based capital, requirement to be well-capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 203,870	$ 215,637
Tier I risk-based capital, excess, percent	0.0307	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 963,840	$ 975,709
Total risk-based capital, capital level, percent	0.1450	0.1469
Total risk-based capital, requirement to be well-capitalized, amount	$ 664,925	$ 664,054
Total risk-based capital, requirement to be well-capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 298,915	$ 311,655
Total risk-based capital, excess, percent	0.0450	0.0469