Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 800,352	$ 733,010
Tier I (leverage) capital, capital level, percent	9.93	9.27
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 403,092	$ 395,510
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00	5.00
Tier I (leverage) capital, excess, amount	$ 397,260	$ 337,500
Tier I (leverage) capital, excess, percent	4.93	4.27
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 800,352	$ 733,010
Common Equity Tier I risk-based capital, capital level, percent	12.91	11.65
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 402,849	$ 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	$ 397,503	$ 324,081
Common Equity Tier I risk-based capital, excess, percent	6.41%	5.15%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 800,352	$ 733,010
Tier I risk-based capital, capital level, percent	12.91	11.65
Tier I risk-based capital, requirement to be well capitalized, amount	$ 495,814	$ 503,297
Tier I risk-based capital, requirement to be well capitalized, percent	8.00	8.00
Tier I risk-based capital, excess, amount	$ 304,538	$ 229,713
Tier I risk-based capital, excess, percent	4.91	3.65
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 834,131	$ 773,807
Total risk-based capital, capital level, percent	13.46	12.30
Total risk-based capital, requirement to be well capitalized, amount	$ 619,768	$ 629,121
Total risk-based capital, requirement to be well capitalized, percent	10.00	10.00
Total risk-based capital, excess, amount	$ 214,363	$ 144,686
Total risk-based capital, excess, percent	3.46	2.30
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 711,276	$ 662,987
Tier I (leverage) capital, capital level, percent	8.83	8.38
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 402,936	$ 395,439
Tier I (leverage) capital, requirement to be well capitalized, percent	5.00	5.00
Tier I (leverage) capital, excess, amount	$ 308,340	$ 267,548
Tier I (leverage) capital, excess, percent	3.83	3.38
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 661,340	$ 621,247
Common Equity Tier I risk-based capital, capital level, percent	10.68	9.88
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 402,623	$ 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	$ 258,717	$ 212,553
Common Equity Tier I risk-based capital, excess, percent	4.18%	3.38%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 711,276	$ 662,987
Tier I risk-based capital, capital level, percent	11.48	10.54
Tier I risk-based capital, requirement to be well capitalized, amount	$ 495,537	$ 503,008
Tier I risk-based capital, requirement to be well capitalized, percent	8.00	8.00
Tier I risk-based capital, excess, amount	$ 215,739	$ 159,979
Tier I risk-based capital, excess, percent	3.48	2.54
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 832,255	$ 794,034
Total risk-based capital, capital level, percent	13.44	12.63
Total risk-based capital, requirement to be well capitalized, amount	$ 619,421	$ 628,760
Total risk-based capital, requirement to be well capitalized, percent	10.00	10.00
Total risk-based capital, excess, amount	$ 212,834	$ 165,274
Total risk-based capital, excess, percent	3.44	2.63