Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 906,437	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1055	0.1056
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 429,793	$ 433,667
Tier I (leverage) capital, requirement to be well capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 476,644	$ 481,961
Tier I (leverage) capital, excess, percent	0.0555	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 906,437	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1361	0.1379
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 433,017	$ 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	$ 473,420	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0711%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 906,437	$ 915,628
Tier I risk-based capital, capital level, percent	0.1361	0.1379
Tier I risk-based capital, requirement to be well capitalized, amount	$ 532,944	$ 531,365
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 373,493	$ 384,263
Tier I risk-based capital, excess, percent	0.0561	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 944,683	$ 954,457
Total risk-based capital, capital level, percent	0.1418	0.1437
Total risk-based capital, requirement to be well capitalized, amount	$ 666,181	$ 664,206
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 278,502	$ 290,251
Total risk-based capital, excess, percent	0.0418	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 737,138	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0858	0.0861
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 429,761	$ 433,607
Tier I (leverage) capital, requirement to be well capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 307,377	$ 313,273
Tier I (leverage) capital, excess, percent	0.0358	0.0361
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 690,846	$ 698,258
Common Equity Tier I risk-based capital, capital level, percent	0.1037	0.1052
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 432,870	$ 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	$ 257,976	$ 266,623
Common Equity Tier I risk-based capital, excess, percent	0.0387%	0.0402%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 737,138	$ 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	$ 532,763	$ 531,243
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 204,375	$ 215,637
Tier I risk-based capital, excess, percent	0.0307	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 965,384	$ 975,709
Total risk-based capital, capital level, percent	0.1450	0.1469
Total risk-based capital, requirement to be well capitalized, amount	$ 665,953	$ 664,054
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 299,431	$ 311,655
Total risk-based capital, excess, percent	0.0450	0.0469