Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative assets
Available-for-sale investment securities	$ 1,298,010	$ 1,388,533
Corporate bonds
Derivative assets
Available-for-sale investment securities	51,151	49,132
Mortgage revenue bonds
Derivative assets
Available-for-sale investment securities	28,166	23,636
Fair value measurements on a recurring basis | Level 1
Derivative liabilities
Derivative liabilities	78	34
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	78	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	2,066	596
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,269,844	1,364,897
Derivative assets	483	91
Fair value measurements on a recurring basis | Level 2 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	473	91
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Derivative assets
Derivative assets	10	0
Derivative liabilities
Derivative liabilities	47	9
Fair value measurements on a recurring basis | Level 2 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities	2,019	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	1,087,487	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	131,206	154,349
Fair value measurements on a recurring basis | Level 2 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	51,151	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	28,166	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
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	$ 28,166	$ 23,636