Segment reporting - Schedule of Segment Financial Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 57,023,000	$ 51,517,000	$ 110,039,000	$ 99,946,000
Provision for loan losses	881,000	1,063,000	2,272,000	1,380,000
Mortgage banking income	28,326,000	30,322,000	51,228,000	58,506,000
Change in fair value of mortgage servicing rights	(3,800,000)	(1,778,000)	(5,681,000)	(3,491,000)
Other noninterest income	8,453,000	7,219,000	16,471,000	14,023,000
Depreciation and amortization	1,278,000	1,132,000	2,450,000	2,238,000
Amortization of intangibles	1,254,000	802,000	1,983,000	1,655,000
Other noninterest mortgage banking expense	21,863,000	25,089,000	42,050,000	48,968,000
Other noninterest expense	39,724,000	29,335,000	72,737,000	59,648,000
Income before income taxes	25,002,000	29,859,000	50,565,000	55,095,000
Income tax expense	6,314,000	7,794,000	12,289,000	13,276,000
Net income	18,688,000	22,065,000	38,276,000	41,819,000
Total assets	5,940,402,000	4,923,249,000	5,940,402,000	4,923,249,000	$ 5,136,764,000
Goodwill	168,486,000	137,190,000	168,486,000	137,190,000	$ 137,190,000
Merger costs	3,783,000	0	4,404,000	1,193,000
Impairment of goodwill			100,000	0
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	56,979,000	51,669,000	109,972,000	100,440,000
Provision for loan losses	881,000	1,063,000	2,272,000	1,380,000
Mortgage banking income	5,451,000	6,894,000	9,837,000	13,002,000
Change in fair value of mortgage servicing rights	0	0	0	0
Other noninterest income	8,453,000	7,219,000	16,471,000	14,023,000
Depreciation and amortization	1,134,000	990,000	2,176,000	1,968,000
Amortization of intangibles	1,254,000	802,000	1,983,000	1,655,000
Other noninterest mortgage banking expense	4,172,000	5,649,000	7,003,000	10,746,000
Other noninterest expense	38,895,000	29,335,000	70,854,000	59,648,000
Income before income taxes	24,547,000	27,943,000	51,992,000	52,068,000
Total assets	5,552,893,000	4,443,469,000	5,552,893,000	4,443,469,000
Goodwill	168,486,000	137,090,000	168,486,000	137,090,000
Merger costs	3,783,000		4,404,000
Offering cost		671,000		671,000
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	44,000	(152,000)	67,000	(494,000)
Provision for loan losses	0	0	0	0
Mortgage banking income	22,875,000	23,428,000	41,391,000	45,504,000
Change in fair value of mortgage servicing rights	(3,800,000)	(1,778,000)	(5,681,000)	(3,491,000)
Other noninterest income	0	0	0	0
Depreciation and amortization	144,000	142,000	274,000	270,000
Amortization of intangibles	0	0	0	0
Other noninterest mortgage banking expense	17,691,000	19,440,000	35,047,000	38,222,000
Other noninterest expense	829,000	0	1,883,000	0
Income before income taxes	455,000	1,916,000	(1,427,000)	3,027,000
Total assets	387,509,000	479,780,000	387,509,000	479,780,000
Goodwill	0	$ 100,000	0	$ 100,000
Impairment of goodwill	$ 100		$ 100,000