Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 840,105	$ 733,010
Tier I (leverage) capital, capital level, percent	10.39	9.27
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 404,366	$ 395,510
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00	5.00
Tier I (leverage) capital, excess, amount	$ 435,739	$ 337,500
Tier I (leverage) capital, excess, percent	5.39	4.27
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 840,105	$ 733,010
Common Equity Tier I risk-based capital, capital level, percent	13.58	11.65
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 402,100	$ 408,929
Common Equity Tier I risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common Equity Tier I risk-based capital, excess, amount	$ 438,005	$ 324,081
Common Equity Tier I risk-based capital, excess, percent	7.08%	5.15%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 840,105	$ 733,010
Tier I risk-based capital, capital level, percent	13.58	11.65
Tier I risk-based capital, requirement to be well capitalized, amount	$ 494,892	$ 503,297
Tier I risk-based capital, requirement to be well capitalized, percent	8.00	8.00
Tier I risk-based capital, excess, amount	$ 345,213	$ 229,713
Tier I risk-based capital, excess, percent	5.58	3.65
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 874,400	$ 773,807
Total risk-based capital, capital level, percent	14.13	12.30
Total risk-based capital, requirement to be well capitalized, amount	$ 618,615	$ 629,121
Total risk-based capital, requirement to be well capitalized, percent	10.00	10.00
Total risk-based capital, excess, amount	$ 255,785	$ 144,686
Total risk-based capital, excess, percent	4.13	2.30
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 726,174	$ 662,987
Tier I (leverage) capital, capital level, percent	8.98	8.38
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 404,422	$ 395,439
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00	5.00
Tier I (leverage) capital, excess, amount	$ 321,752	$ 267,548
Tier I (leverage) capital, excess, percent	3.98	3.38
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 671,494	$ 621,247
Common Equity Tier I risk-based capital, capital level, percent	10.86	9.88
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 401,836	$ 408,694
Common Equity Tier I risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common Equity Tier I risk-based capital, excess, amount	$ 269,658	$ 212,553
Common Equity Tier I risk-based capital, excess, percent	4.36%	3.38%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 726,174	$ 662,987
Tier I risk-based capital, capital level, percent	11.75	10.54
Tier I risk-based capital, requirement to be well capitalized, amount	$ 494,568	$ 503,008
Tier I risk-based capital, requirement to be well capitalized, percent	8.00	8.00
Tier I risk-based capital, excess, amount	$ 231,606	$ 159,979
Tier I risk-based capital, excess, percent	3.75	2.54
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 885,469	$ 794,034
Total risk-based capital, capital level, percent	14.32	12.63
Total risk-based capital, requirement to be well capitalized, amount	$ 618,210	$ 628,760
Total risk-based capital, requirement to be well capitalized, percent	10.00	10.00
Total risk-based capital, excess, amount	$ 267,259	$ 165,274
Total risk-based capital, excess, percent	4.32	2.63