Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 55,337	$ 57,023	$ 111,586	$ 110,039
Provisions for credit losses	25,921	881	55,486	2,272
Mortgage banking income	85,406	28,326	124,019	51,228
Change in fair value of mortgage servicing rights, net of hedging	(13,238)	(3,800)	(19,106)	(5,681)
Other noninterest income	9,323	8,453	19,278	16,471
Depreciation and amortization	1,619	1,278	3,231	2,450
Amortization of intangibles	1,205	1,254	2,408	1,983
Other noninterest mortgage banking expense	38,423	21,863	63,045	42,050
Other noninterest expense	39,332	39,724	80,454	72,737
Income before income taxes	30,328	25,002	31,153	50,565
Income tax expense	7,455	6,314	7,535	12,289
Net income	22,873	18,688	23,618	38,276
Total assets	7,255,536	5,940,402	7,255,536	5,940,402	$ 6,124,921
Goodwill	175,441	168,486	175,441	168,486	$ 169,051	$ 137,190
Provision for credit losses on unfunded commitments	1,882	0	3,483	0
Merger costs	1,586	3,783	4,636	4,404
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	55,350	56,979	111,583	109,972
Provisions for credit losses	25,921	881	55,486	2,272
Mortgage banking income	16,940	5,451	27,591	9,837
Change in fair value of mortgage servicing rights, net of hedging	0	0	0	0
Other noninterest income	9,323	8,453	19,278	16,471
Depreciation and amortization	1,503	1,134	2,995	2,176
Amortization of intangibles	1,205	1,254	2,408	1,983
Other noninterest mortgage banking expense	11,542	4,172	18,717	7,003
Other noninterest expense	39,332	38,895	80,454	70,854
Income before income taxes	2,110	24,547	(1,608)	51,992
Total assets	6,751,881	5,552,893	6,751,881	5,552,893
Goodwill	175,441	168,486	175,441	168,486
Merger costs	1,586	3,783	4,636
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	(13)	44	3	67
Provisions for credit losses	0	0	0	0
Mortgage banking income	68,466	22,875	96,428	41,391
Change in fair value of mortgage servicing rights, net of hedging	(13,238)	(3,800)	(19,106)	(5,681)
Other noninterest income	0	0	0	0
Depreciation and amortization	116	144	236	274
Amortization of intangibles	0	0	0	0
Other noninterest mortgage banking expense	26,881	17,691	44,328	35,047
Other noninterest expense	0	829	0	1,883
Income before income taxes	28,218	455	32,761	(1,427)
Total assets	503,655	387,509	503,655	387,509
Goodwill	$ 0	$ 0	$ 0	$ 0