Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Savings bank
Banking Regulation, Tier 1 Leverage Capital [Abstract]
Banking Regulation, Tier 1 Leverage Capital, Actual	$ 895,465	$ 847,588
Banking Regulation, Tier 1 Leverage Capital Ratio, Actual	0.103	0.0931
Banking Regulation, Tier 1 Leverage Capital, Well Capitalized, Minimum	$ 434,841	$ 455,335
Banking Regulation, Tier 1 Leverage Capital Ratio, Well Capitalized, Minimum	0.05	0.05
Banking Regulation, Excess Tier 1 Leverage Capital, Actual	$ 460,624	$ 392,253
Banking Regulation, Excess Tier 1 Leverage Capital Ratio, Actual	0.053	0.0431
Common Equity Tier I risk-based capital:
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Actual	$ 895,465	$ 847,588
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1338	0.1251
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 435,047	$ 440,259
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	$ 460,418	$ 407,329
Common Equity Tier I risk-based capital, excess, percent	0.0688%	0.0601%
Tier I risk-based capital:
Banking Regulation, Tier 1 Risk-Based Capital, Actual	$ 895,465	$ 847,588
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Actual	0.1338	0.1251
Banking Regulation, Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 535,442	$ 541,857
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Well Capitalized, Minimum	0.08	0.08
Banking Regulation, Tier 1 Risk-Based Capital, Excess, Actual	$ 360,023	$ 305,731
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Excess, Actual	0.0538	0.0451
Banking Regulation, Total Capital [Abstract]
Banking Regulation, Total Capital, Actual	$ 938,033	$ 887,902
Banking Regulation, Total Risk-Based Capital Ratio, Actual	0.1402	0.1311
Banking Regulation, Total Risk-Based Capital, Well Capitalized, Minimum	$ 669,303	$ 677,321
Banking Regulation, Total Risk-Based Capital Ratio, Well Capitalized, Minimum	0.10	0.10
Banking Regulation, Total Risk-Based Capital, Excess, Actual	$ 268,730	$ 210,581
Banking Regulation, Total Risk-Based Capital Ratio, Excess, Actual	0.0402	0.0311
Holding company
Banking Regulation, Tier 1 Leverage Capital [Abstract]
Banking Regulation, Tier 1 Leverage Capital, Actual	$ 751,258	$ 731,958
Banking Regulation, Tier 1 Leverage Capital Ratio, Actual	0.0864	0.0804
Banking Regulation, Tier 1 Leverage Capital, Well Capitalized, Minimum	$ 434,790	$ 455,297
Banking Regulation, Tier 1 Leverage Capital Ratio, Well Capitalized, Minimum	0.05	0.05
Banking Regulation, Excess Tier 1 Leverage Capital, Actual	$ 316,468	$ 276,661
Banking Regulation, Excess Tier 1 Leverage Capital Ratio, Actual	0.0364	0.0304
Common Equity Tier I risk-based capital:
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Actual	$ 703,450	$ 685,004
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1051	0.1013
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 434,982	$ 439,533
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	$ 268,468	$ 245,471
Common Equity Tier I risk-based capital, excess, percent	0.0401%	0.0363%
Tier I risk-based capital:
Banking Regulation, Tier 1 Risk-Based Capital, Actual	$ 751,258	$ 731,958
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Actual	0.1123	0.1082
Banking Regulation, Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 535,363	$ 540,964
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Well Capitalized, Minimum	0.08	0.08
Banking Regulation, Tier 1 Risk-Based Capital, Excess, Actual	$ 215,895	$ 190,994
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Excess, Actual	0.0323	0.0282
Banking Regulation, Total Capital [Abstract]
Banking Regulation, Total Capital, Actual	$ 983,826	$ 962,272
Banking Regulation, Total Risk-Based Capital Ratio, Actual	0.147	0.1423
Banking Regulation, Total Risk-Based Capital, Well Capitalized, Minimum	$ 669,204	$ 676,205
Banking Regulation, Total Risk-Based Capital Ratio, Well Capitalized, Minimum	0.10	0.10
Banking Regulation, Total Risk-Based Capital, Excess, Actual	$ 314,622	$ 286,067
Banking Regulation, Total Risk-Based Capital Ratio, Excess, Actual	0.047	0.0423

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

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