Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative assets
Available-for-sale investment securities	$ 1,418,490	$ 1,401,198
Fair value measurements on a recurring basis | Level 1
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	60	20
Fair value measurements on a recurring basis | Level 1 | 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 | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	60	20
Fair value measurements on a recurring basis | Level 1 | Window forward contracts
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 1 | Bank
Derivative assets
Available-for-sale investment securities	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 | Mortgage revenue bond | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 2
Derivative assets
Derivative assets	611	393
Derivative liabilities
Derivative liabilities	27	10
Fair value measurements on a recurring basis | Level 2 | Interest rate lock commitments
Derivative assets
Derivative assets	264	133
Derivative liabilities
Derivative liabilities	9	2
Fair value measurements on a recurring basis | Level 2 | Forward commitments
Derivative assets
Derivative assets	18	4
Derivative liabilities
Derivative liabilities	18	8
Fair value measurements on a recurring basis | Level 2 | Window forward contracts
Derivative assets
Derivative assets	329	256
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,403,063	1,385,771
Fair value measurements on a recurring basis | Level 2 | Mortgage-related securities - FNMA, FHLMC and GNMA | Bank
Derivative assets
Available-for-sale investment securities	1,224,235	1,201,473
Fair value measurements on a recurring basis | Level 2 | U.S. Treasury federal agency obligations | Bank
Derivative assets
Available-for-sale investment securities	178,828	184,298
Fair value measurements on a recurring basis | Level 2 | Mortgage revenue bond | Bank
Derivative assets
Available-for-sale investment securities	0	0
Fair value measurements on a recurring basis | Level 3
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | 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 | Forward commitments
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Window forward contracts
Derivative assets
Derivative assets	0	0
Fair value measurements on a recurring basis | Level 3 | Bank
Derivative assets
Available-for-sale investment securities	15,427	15,427
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 | Mortgage revenue bond | Bank
Derivative assets
Available-for-sale investment securities	$ 15,427	$ 15,427