Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative assets
Available-for-sale investment securities	$ 1,210,748	$ 1,388,533
Corporate bonds
Derivative assets
Available-for-sale investment securities	36,276	49,132
Mortgage revenue bonds
Derivative assets
Available-for-sale investment securities	28,459	23,636
Fair value measurements on a recurring basis | Level 1
Derivative liabilities
Derivative liabilities	18	34
Fair value measurements on a recurring basis | Level 1 | Bank
Derivative assets
Available-for-sale investment securities	0	0
Derivative assets	2	0
Fair value measurements on a recurring basis | Level 1 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Forward commitments
Derivative assets
Derivative assets	2	0
Derivative liabilities
Derivative liabilities	18	34
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	2,901	596
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,182,289	1,364,897
Derivative assets	484	91
Fair value measurements on a recurring basis | Level 2 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	477	91
Derivative liabilities
Derivative liabilities	7	0
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Derivative assets
Derivative assets	7	0
Derivative liabilities
Derivative liabilities	67	9
Fair value measurements on a recurring basis | Level 2 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities	2,827	587
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	1,019,305	1,161,416
Fair value measurements on a recurring basis | Level 2 | U.S. Treasury and federal agency obligations | Bank
Derivative assets
Available-for-sale investment securities	126,708	154,349
Fair value measurements on a recurring basis | Level 2 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	36,276	49,132
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	28,459	23,636
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	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	$ 28,459	$ 23,636