Segment reporting - Additional Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Other noninterest expense	$ 39,332	$ 39,724	$ 80,454	$ 72,737
Relief of goodwill due to sale of TPO mortgage delivery channel			0	100
Interest income	61,092	66,276	124,846	126,724
Mortgage Segment
Segment Reporting Information [Line Items]
Other noninterest expense	0	829	0	1,883
Relief of goodwill due to sale of TPO mortgage delivery channel		100		100
Interest paid	$ 3,335	$ 3,290	$ 5,710	$ 5,848
Mortgage Segment | Prime Interest Rate
Segment Reporting Information [Line Items]
Warehouse line of credit interest rate	3.25%	5.50%	3.25%	5.50%
Banking Segment
Segment Reporting Information [Line Items]
Other noninterest expense	$ 39,332	$ 38,895	$ 80,454	$ 70,854
Interest income	$ 3,335	$ 3,290	$ 5,710	$ 5,848