Fair value measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative assets
Available-for-sale investment securities	$ 1,387,571	$ 1,401,198
Corporate bonds
Derivative assets
Available-for-sale investment securities	49,383
Mortgage revenue bonds
Derivative assets
Available-for-sale investment securities	19,084	15,427
Fair value measurements on a recurring basis | Level 1
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	0	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	0	20
Fair value measurements on a recurring basis | Level 1 | Window forward contracts
Derivative assets
Derivative assets	0	0
Derivative liabilities
Derivative liabilities	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 | 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 assets
Derivative assets	0	393
Derivative liabilities
Derivative liabilities	27	10
Fair value measurements on a recurring basis | Level 2 | Interest rate lock commitments
Derivative assets
Derivative assets	0	133
Derivative liabilities
Derivative liabilities	0	2
Fair value measurements on a recurring basis | Level 2 | Forward commitments
Derivative assets
Derivative assets	0	4
Derivative liabilities
Derivative liabilities	0	8
Fair value measurements on a recurring basis | Level 2 | Window forward contracts
Derivative assets
Derivative assets	0	256
Derivative liabilities
Derivative liabilities	27	0
Fair value measurements on a recurring basis | Level 2 | Bank
Derivative assets
Available-for-sale investment securities	1,368,487	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	1,148,690	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	170,414	184,298
Fair value measurements on a recurring basis | Level 2 | Corporate bonds | Bank
Derivative assets
Available-for-sale investment securities	49,383	0
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 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
Derivative liabilities
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Bank
Derivative assets
Available-for-sale investment securities	19,084	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 | 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	$ 19,084	$ 15,427