Segment reporting - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Other noninterest expense	$ 146,653	$ 121,200	$ 117,461
Mortgage Segment
Segment Reporting Information [Line Items]
Other noninterest expense	1,995	0	0
Interest paid	$ 11,183	$ 16,057	$ 16,932
Mortgage Segment | Prime Interest Rate
Segment Reporting Information [Line Items]
Warehouse line of credit interest rate	4.75%	5.50%	4.50%
Banking Segment
Segment Reporting Information [Line Items]
Other noninterest expense	$ 144,658	$ 121,200	$ 117,461