Fair Value Measurements - Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	$ 1,171,430	$ 1,138,352
Trading securities	10,993	11,222
Impaired loans	6,113	4,942
Measured on a recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	1,171,430	1,138,352
Trading securities	10,993	11,222
Equity securities	4,502	250
Total	1,186,925	1,149,824
Measured on a recurring basis | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	3,170
Measured on a recurring basis | Mortgage-backed securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	1,069,182	973,127
Measured on a recurring basis | Pass-through certificates, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	312,051	329,407
Measured on a recurring basis | REMICs, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	753,955	643,667
Measured on a recurring basis | Non-GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	6	53
Measured on a recurring basis | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	229	299
Measured on a recurring basis | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	101,175	164,926
Measured on a recurring basis | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	844
Measured on a recurring basis | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	102,248	165,225
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Trading securities	10,993	11,222
Equity securities	4,502	250
Total	15,495	11,472
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pass-through certificates, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | REMICs, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	1,171,430	1,138,352
Trading securities	0	0
Equity securities	0	0
Total	1,171,430	1,138,352
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	3,170
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	1,069,182	973,127
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Pass-through certificates, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	312,051	329,407
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | REMICs, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	753,955	643,667
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Non-GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	6	53
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	229	299
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	101,175	164,926
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	844
Measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	102,248	165,225
Measured on a recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Trading securities	0	0
Equity securities	0	0
Total	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Pass-through certificates, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | REMICs, GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Non-GSE
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0
Measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available-for-sale	0	0
Measured on a non-recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,113	4,942
Measured on a non-recurring basis | Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	6,100	4,927
Measured on a non-recurring basis | Real Estate | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	6,052	4,871
Measured on a non-recurring basis | Real Estate | Multifamily
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	19	26
Measured on a non-recurring basis | Real Estate | Home Equity and Lines of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	29	30
Measured on a non-recurring basis | Commercial and Industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	13	15
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real Estate | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real Estate | Multifamily
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real Estate | Home Equity and Lines of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial and Industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2) | Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2) | Real Estate | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2) | Real Estate | Multifamily
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2) | Real Estate | Home Equity and Lines of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Other Observable Inputs (Level 2) | Commercial and Industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0	0
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,113	4,942
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3) | Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	6,100	4,927
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3) | Real Estate | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	6,052	4,871
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3) | Real Estate | Multifamily
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	19	26
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3) | Real Estate | Home Equity and Lines of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	29	30
Measured on a non-recurring basis | Significant Unobservable Inputs (Level 3) | Commercial and Industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 13	$ 15