Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative assets
Available-for-sale investment securities	$ 1,348,263	$ 1,388,533
Corporate bonds
Derivative assets
Available-for-sale investment securities	50,462	49,132
Mortgage revenue bonds
Derivative assets
Available-for-sale investment securities	27,970	23,636
Fair value measurements on a recurring basis | Level 1
Derivative liabilities
Derivative liabilities	141	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
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	141	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-related securities - FNMA, FHLMC and GNMA | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 1 | U.S. Treasury 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	1,160	596
Fair value measurements on a recurring basis | Level 2 | Interest rate swap
Derivative liabilities
Derivative liabilities	1,130
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,320,293	1,364,897
Derivative assets	472	91
Fair value measurements on a recurring basis | Level 2 | Bank | Interest rate lock commitments
Derivative assets
Derivative assets	463	91
Derivative liabilities
Derivative liabilities	1	0
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Derivative assets
Derivative assets	9	0
Derivative liabilities
Derivative liabilities	29	9
Fair value measurements on a recurring basis | Level 2 | Other | Interest rate swap
Derivative liabilities
Derivative liabilities		587
Fair value measurements on a recurring basis | Level 2 | Mortgage-related securities - FNMA, FHLMC and GNMA | Bank
Derivative assets
Available-for-sale investment securities	1,128,987	1,161,416
Fair value measurements on a recurring basis | Level 2 | U.S. Treasury federal agency obligations | Bank
Derivative assets
Available-for-sale investment securities	140,844	154,349
Fair value measurements on a recurring basis | Level 2 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	50,462	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	27,970	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-related securities - FNMA, FHLMC and GNMA | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 3 | U.S. Treasury 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	$ 27,970	$ 23,636