Segment reporting - Schedule of Segment Financial Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 58,305,000	$ 52,755,000	$ 168,344,000	$ 152,701,000
Provision for loan losses	1,831,000	1,818,000	4,103,000	3,198,000
Mortgage banking income	34,284,000	29,350,000	85,512,000	87,856,000
Change in fair value of mortgage servicing rights	(5,091,000)	(2,701,000)	(10,772,000)	(6,192,000)
Other noninterest income	8,952,000	7,706,000	25,423,000	21,729,000
Depreciation and amortization	1,380,000	1,013,000	3,830,000	3,251,000
Amortization of intangibles	1,197,000	777,000	3,180,000	2,432,000
Other noninterest mortgage banking expense	23,648,000	25,087,000	65,698,000	74,055,000
Other noninterest expense	36,710,000	30,336,000	109,447,000	89,984,000
Income before income taxes	31,684,000	28,079,000	82,249,000	83,174,000
Income tax expense	7,718,000	6,702,000	20,007,000	19,978,000
Net income	23,966,000	21,377,000	62,242,000	63,196,000
Total assets	6,088,895,000	5,058,167,000	6,088,895,000	5,058,167,000	$ 5,136,764,000
Goodwill	168,486,000	137,190,000	168,486,000	137,190,000	$ 137,190,000
Merger costs	295,000	0	4,699,000	1,193,000
Impairment of goodwill			100,000	0
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	58,350,000	52,733,000	168,322,000	153,173,000
Provision for loan losses	1,831,000	1,818,000	4,103,000	3,198,000
Mortgage banking income	10,693,000	7,417,000	20,530,000	20,419,000
Change in fair value of mortgage servicing rights	0	0	0	0
Other noninterest income	8,952,000	7,706,000	25,423,000	21,729,000
Depreciation and amortization	1,255,000	896,000	3,431,000	2,864,000
Amortization of intangibles	1,197,000	777,000	3,180,000	2,432,000
Other noninterest mortgage banking expense	8,087,000	6,383,000	15,090,000	17,129,000
Other noninterest expense	36,598,000	30,336,000	107,452,000	89,984,000
Income before income taxes	29,027,000	27,646,000	81,019,000	79,714,000
Total assets	5,730,492,000	4,637,097,000	5,730,492,000	4,637,097,000
Goodwill	168,486,000	137,090,000	168,486,000	137,090,000
Merger costs	295,000		4,699,000
Offering cost				671,000
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	(45,000)	22,000	22,000	(472,000)
Provision for loan losses	0	0	0	0
Mortgage banking income	23,591,000	21,933,000	64,982,000	67,437,000
Change in fair value of mortgage servicing rights	(5,091,000)	(2,701,000)	(10,772,000)	(6,192,000)
Other noninterest income	0	0	0	0
Depreciation and amortization	125,000	117,000	399,000	387,000
Amortization of intangibles	0	0	0	0
Other noninterest mortgage banking expense	15,561,000	18,704,000	50,608,000	56,926,000
Other noninterest expense	112,000	0	1,995,000	0
Income before income taxes	2,657,000	433,000	1,230,000	3,460,000
Total assets	358,403,000	421,070,000	358,403,000	421,070,000
Goodwill	$ 0	$ 100,000	0	$ 100,000
Impairment of goodwill			$ 100,000