Fair value measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Fair value measurements on a recurring basis
Available-for-sale investment securities	$ 1,084,083	$ 1,136,439
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Derivative assets
Derivative assets	10	0
Derivative liabilities
Derivative liabilities	0	18
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Bank | Forward commitments
Derivative assets
Derivative assets	10	0
Derivative liabilities
Derivative liabilities	0	18
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Interest rate swap (Other segment) | Interest rate swap
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. Treasury and federal agency obligations | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Mortgage revenue bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2)
Derivative assets
Derivative assets	792	1,058
Derivative liabilities
Derivative liabilities	116	303
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	1,070,148	1,122,081
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	68	86
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Interest rate swap (Other segment) | Interest rate swap
Derivative assets
Derivative assets	724	972
Derivative liabilities
Derivative liabilities	116	303
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	1,028,627	1,077,168
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | U.S. Treasury and federal agency obligations | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	7,597	12,010
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Corporate bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	33,924	32,903
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Mortgage revenue bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3)
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	13,935	14,358
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Bank | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Interest rate swap (Other segment) | Interest rate swap
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | U.S. Treasury and federal agency obligations | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Corporate bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Mortgage revenue bonds | Bank
Fair value measurements on a recurring basis
Available-for-sale investment securities	$ 13,935	$ 14,358