Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 56,249	$ 53,016
Provisions for credit losses	29,565	1,391
Mortgage banking income	38,613	22,902
Change in fair value of mortgage servicing rights, net of hedging	(5,868)	(1,881)
Other noninterest income	9,955	8,018
Depreciation and amortization	1,612	1,172
Amortization of intangibles	1,203	729
Other noninterest mortgage banking expense	24,622	20,187
Other noninterest expense	41,122	33,013
Income before income taxes	825	25,563
Income tax expense	80	5,975
Net income	745	19,588
Total assets	6,655,687	5,335,156	$ 6,124,921
Goodwill	174,859	137,190	$ 169,051
Provision for credit losses on unfunded commitments	1,601	0
Merger costs	3,050	621
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	56,233	52,993
Provisions for credit losses	29,565	1,391
Mortgage banking income	10,651	4,386
Change in fair value of mortgage servicing rights, net of hedging	0	0
Other noninterest income	9,955	8,018
Depreciation and amortization	1,492	1,042
Amortization of intangibles	1,203	729
Other noninterest mortgage banking expense	7,175	2,831
Other noninterest expense	41,122	31,959
Income before income taxes	(3,718)	27,445
Total assets	6,211,640	4,987,744
Goodwill	174,859	137,090
Merger costs	3,050	621
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	16	23
Provisions for credit losses	0	0
Mortgage banking income	27,962	18,516
Change in fair value of mortgage servicing rights, net of hedging	(5,868)	(1,881)
Other noninterest income	0	0
Depreciation and amortization	120	130
Amortization of intangibles	0	0
Other noninterest mortgage banking expense	17,447	17,356
Other noninterest expense	0	1,054
Income before income taxes	4,543	(1,882)
Total assets	444,047	347,412
Goodwill	$ 0	$ 100