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	$ 905,323	$ 915,628
Tier I (leverage) capital, capital level, percent	0.1053	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	$ 475,530	$ 481,961
Tier I (leverage) capital, excess, percent	0.0553	0.0556
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 905,323	$ 915,628
Common Equity Tier I risk-based capital, capital level, percent	0.1359	0.1379
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 433,057	$ 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	$ 472,266	$ 483,894
Common Equity Tier I risk-based capital, excess, percent	0.0709%	0.0729%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 905,323	$ 915,628
Tier I risk-based capital, capital level, percent	0.1359	0.1379
Tier I risk-based capital, requirement to be well capitalized, amount	$ 532,993	$ 531,365
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 372,330	$ 384,263
Tier I risk-based capital, excess, percent	0.0559	0.0579
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 943,569	$ 954,457
Total risk-based capital, capital level, percent	0.1416	0.1437
Total risk-based capital, requirement to be well capitalized, amount	$ 666,242	$ 664,206
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 277,327	$ 290,251
Total risk-based capital, excess, percent	0.0416	0.0437
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 736,024	$ 746,880
Tier I (leverage) capital, capital level, percent	0.0856	0.0861
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 429,762	$ 433,607
Tier I (leverage) capital, requirement to be well capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 306,622	$ 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	$ 689,732	$ 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	$ 432,909	$ 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	$ 256,823	$ 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	$ 736,024	$ 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	$ 532,812	$ 531,243
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 203,212	$ 215,637
Tier I risk-based capital, excess, percent	0.0305	0.0325
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 964,270	$ 975,709
Total risk-based capital, capital level, percent	0.1448	0.1469
Total risk-based capital, requirement to be well capitalized, amount	$ 666,015	$ 664,054
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 298,255	$ 311,655
Total risk-based capital, excess, percent	0.0448	0.0469