Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net interest income	$ 226,036	$ 204,068	$ 153,271
Provision for loan loss	7,053	5,398	(950)
Mortgage banking income	117,905	109,334	120,357
Change in fair value of mortgage servicing rights, net of hedging	(16,989)	(8,673)	(3,424)
Other noninterest income	34,481	29,981	24,648
Depreciation and amortization	5,176	4,334	4,316
Amortization of intangibles	4,339	3,185	1,995
Loss on sale of mortgage servicing rights	0	0	249
Other noninterest mortgage banking expense	88,673	94,739	98,296
Other noninterest expense	146,653	121,200	117,461
Income before income taxes	109,539	105,854	73,485
Income tax expense	25,725	25,618	21,087
Net income	83,814	80,236	52,398
Total assets	6,124,921	5,136,764	4,727,713
Goodwill	169,051	137,190	137,190
Merger costs	5,385	1,594	19,034
Relief of goodwill	100	0	0
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	226,098	204,517	153,018
Provision for loan loss	7,053	5,398	(950)
Mortgage banking income	30,429	25,460	26,737
Change in fair value of mortgage servicing rights, net of hedging	0	0	0
Other noninterest income	34,481	29,981	24,648
Depreciation and amortization	4,670	3,827	3,801
Amortization of intangibles	4,339	3,185	1,995
Loss on sale of mortgage servicing rights			0
Other noninterest mortgage banking expense	23,216	21,671	21,714
Other noninterest expense	144,658	121,200	117,461
Income before income taxes	107,072	104,677	60,382
Total assets	5,795,888	4,752,111	4,130,349
Goodwill	169,051	137,090	137,090
Merger costs	5,385
Offering cost		671
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	(62)	(449)	253
Provision for loan loss	0	0	0
Mortgage banking income	87,476	83,874	93,620
Change in fair value of mortgage servicing rights, net of hedging	(16,989)	(8,673)	(3,424)
Other noninterest income	0	0	0
Depreciation and amortization	506	507	515
Amortization of intangibles	0	0	0
Loss on sale of mortgage servicing rights			249
Other noninterest mortgage banking expense	65,457	73,068	76,582
Other noninterest expense	1,995	0	0
Income before income taxes	2,467	1,177	13,103
Total assets	329,033	384,653	597,364
Goodwill	0	$ 100	$ 100
Relief of goodwill	$ 100