Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 53,016	$ 48,429
Provision for loan losses	1,391	317
Mortgage banking income	22,902	28,184
Change in fair value of mortgage servicing rights	(1,881)	(1,713)
Other noninterest income	8,018	6,804
Depreciation and amortization	1,172	1,106
Amortization of intangibles	729	853
Other noninterest mortgage banking expense	20,187	23,879
Other noninterest expense	33,013	30,313
Income before income taxes	25,563	25,236
Income tax expense	5,975	5,482
Net income	19,588	19,754
Total assets	5,335,156	4,725,416	$ 5,136,764
Goodwill	137,190	137,190	$ 137,190
Merger costs	621	1,193
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	52,993	48,771
Provision for loan losses	1,391	317
Mortgage banking income	4,386	6,108
Change in fair value of mortgage servicing rights	0	0
Other noninterest income	8,018	6,804
Depreciation and amortization	1,042	978
Amortization of intangibles	729	853
Other noninterest mortgage banking expense	2,831	5,097
Other noninterest expense	31,959	30,313
Income before income taxes	27,445	24,125
Total assets	4,987,744	4,220,543
Goodwill	137,090	137,090
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	23	(342)
Provision for loan losses	0	0
Mortgage banking income	18,516	22,076
Change in fair value of mortgage servicing rights	(1,881)	(1,713)
Other noninterest income	0	0
Depreciation and amortization	130	128
Amortization of intangibles	0	0
Other noninterest mortgage banking expense	17,356	18,782
Other noninterest expense	1,054	0
Income before income taxes	(1,882)	1,111
Total assets	347,412	504,873
Goodwill	$ 100	$ 100