Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative assets
Available-for-sale investment securities	$ 1,340,241	$ 1,232,826
Corporate bonds
Derivative assets
Available-for-sale investment securities	59,770	60,057
Mortgage revenue bonds
Derivative assets
Available-for-sale investment securities	28,726	28,597
Fair value measurements on a recurring basis | Level 1
Derivative liabilities
Derivative liabilities	585	33
Fair value measurements on a recurring basis | Level 1 | Bank
Derivative assets
Available-for-sale investment securities	0	0
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	585	33
Fair value measurements on a recurring basis | Level 1 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | 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 | 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 | Level 1 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 1 | Mortgage revenue bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 2
Derivative liabilities
Derivative liabilities	4,575	2,185
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,311,515	1,204,229
Derivative assets	1,852	300
Fair value measurements on a recurring basis | Level 2 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	1,852	297
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Derivative assets
Derivative assets	0	3
Derivative liabilities
Derivative liabilities	0	12
Fair value measurements on a recurring basis | Level 2 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities	4,575	2,173
Fair value measurements on a recurring basis | Level 2 | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies | Bank
Derivative assets
Available-for-sale investment securities	1,141,452	1,026,385
Fair value measurements on a recurring basis | Level 2 | U.S. Treasury and federal agency obligations | Bank
Derivative assets
Available-for-sale investment securities	110,293	117,787
Fair value measurements on a recurring basis | Level 2 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	59,770	60,057
Fair value measurements on a recurring basis | Level 2 | Mortgage revenue bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 3
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Bank
Derivative assets
Available-for-sale investment securities	28,726	28,597
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | 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 | 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 | Level 3 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 3 | Mortgage revenue bonds | Bank
Derivative assets
Available-for-sale investment securities	$ 28,726	$ 28,597