Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Derivative assets
Available-for-sale investment securities	$ 2,509,906	$ 1,970,417
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Derivative liabilities
Derivative liabilities	26	500
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Bank
Derivative assets
Available-for-sale investment securities	0	0
Derivative assets	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	0	0
Derivative liabilities
Derivative liabilities	26	500
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other | 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
Derivative assets
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
Derivative assets
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
Derivative assets
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
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2)
Derivative liabilities
Derivative liabilities	3,626	4,530
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank
Derivative assets
Available-for-sale investment securities	2,494,479	1,943,232
Derivative assets	511	4,536
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	371	4,536
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Bank | Forward commitments
Derivative assets
Derivative assets	6	0
Derivative liabilities
Derivative liabilities	21	0
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Other | Interest rate swap
Derivative assets
Derivative assets	134	0
Derivative liabilities
Derivative liabilities	3,605	4,530
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
Derivative assets
Available-for-sale investment securities	2,366,301	1,849,559
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | U.S. Treasury and federal agency obligations | Bank
Derivative assets
Available-for-sale investment securities	96,309	62,322
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	31,869	31,351
Fair value measurements on a recurring basis | Significant  other observable  inputs (Level 2) | Mortgage revenue bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3)
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Bank
Derivative assets
Available-for-sale investment securities	15,427	27,185
Derivative assets	0	0
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) | Other | 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
Derivative assets
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
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Significant unobservable inputs (Level 3) | Mortgage revenue bonds | Bank
Derivative assets
Available-for-sale investment securities	$ 15,427	$ 27,185