Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net interest income	$ 204,068	$ 153,271	$ 110,950
Provision for loan losses	5,398	(950)	(1,479)
Mortgage banking income	109,334	120,357	117,751
Change in fair value of mortgage servicing rights	(8,673)	(3,424)
Other noninterest income	29,981	24,648	26,934
Depreciation and amortization	4,334	4,316	3,995
Amortization of core deposit and other intangibles	3,185	1,995	2,132
Amortization and impairment of mortgage servicing rights			12,999
Loss on sale of mortgage servicing rights	0	249	4,447
Other noninterest mortgage banking expense	94,739	98,296	82,351
Other noninterest expense	121,200	117,461	88,866
Income before income taxes	105,854	73,485	62,324
Income tax expense	25,618	21,087	21,733
Net income	80,236	52,398	40,591
Total assets	5,136,764	4,727,713	3,276,881
Goodwill	137,190	137,190	46,867
Merger and conversion	1,594	19,034	3,268
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	204,517	153,018	112,365
Provision for loan losses	5,398	(950)	(1,479)
Mortgage banking income	25,460	26,737	25,542
Change in fair value of mortgage servicing rights	0	0
Other noninterest income	29,981	24,648	26,934
Depreciation and amortization	3,827	3,801	3,506
Amortization of core deposit and other intangibles	3,185	1,995	2,132
Amortization and impairment of mortgage servicing rights			0
Loss on sale of mortgage servicing rights		0	0
Other noninterest mortgage banking expense	21,671	21,714	16,095
Other noninterest expense	121,200	117,461	88,866
Income before income taxes	104,677	60,382	55,721
Total assets	4,752,111	4,130,349	2,752,773
Goodwill	137,090	137,090	46,767
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	(449)	253	(1,415)
Provision for loan losses	0	0	0
Mortgage banking income	83,874	93,620	92,209
Change in fair value of mortgage servicing rights	(8,673)	(3,424)
Other noninterest income	0	0	0
Depreciation and amortization	507	515	489
Amortization of core deposit and other intangibles	0	0	0
Amortization and impairment of mortgage servicing rights			12,999
Loss on sale of mortgage servicing rights		249	4,447
Other noninterest mortgage banking expense	73,068	76,582	66,256
Other noninterest expense	0	0	0
Income before income taxes	1,177	13,103	6,603
Total assets	384,653	597,364	524,108
Goodwill	$ 100	$ 100	$ 100