Basis of presentation - Additional Information (Details)		3 Months Ended	12 Months Ended
	May 26, 2017 USD ($) $ / shares shares	Jun. 30, 2018 shares	Dec. 31, 2019 USD ($) branch Method	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Class of Stock [Line Items]
Number of full-service branches | branch			68
Proceeds from issuance of common stock, net of offering costs			$ 804,000	$ 1,196,000	$ 153,356,000
Other than temporary impairment			0	0	945,000
Transfers from loans to loans held for sale			$ 7,891,000	11,888,000	0
Adjustments for transfers to loans HFS				349,000
Percent of remaining principal allowed to buy back under GNMA optional repurchase programs			100.00%
Loans individually evaluated for impairment			$ 23,810,000	$ 13,716,000	$ 9,373,000
Effective tax rate, percent			23.50%	24.20%	28.70%
Interest and penalties on uncertain tax positions			$ 0	$ 0	$ 0
Impairment of long-lived assets			$ 0	0
Number of methods to deliver mortgage loans | Method			2
Deposits with Federal Home Loan Banks			$ 11,225,000	5,225,000
FHLB lender risk account guaranty			5,546,000	2,646,000
Advertising expense			9,138,000	13,139,000	12,957,000
Operating lease right-of-use assets			32,539,000	0
Operating lease liabilities			35,525,000	0
Cumulative effect of new accounting principal						$ 1,309,000	$ 0
Leasehold improvements			90,131,000	86,882,000
Finite lived intangible assets			$ 17,589,000	11,628,000
Accounting Standards Update 2016-02
Class of Stock [Line Items]
One-time adjustment to retained earnings					615,000
Operating lease right-of-use assets						32,545,000
Operating lease liabilities						34,876,000
Private Placement
Class of Stock [Line Items]
Common stock sold and issued, shares (in shares) | shares	4,806,710
Common stock sold and issued, price per share (USD per share) | $ / shares	$ 33.00
Placement agent and other offering costs	$ 5,901,000
Proceeds from issuance of common stock, net of offering costs	$ 152,721,000
Secondary Public Offering
Class of Stock [Line Items]
Common stock sold and issued, shares (in shares) | shares		3,680,000
Retained earnings
Class of Stock [Line Items]
Cumulative effect of new accounting principal						1,309,000	$ 109,000
Retained earnings | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Cumulative effect of new accounting principal						1,309,000
Cumulative effect of new accounting principle in period of adoption, net of deferred taxes						461,000
Premises
Class of Stock [Line Items]
Property, plant and equipment, useful life			40 years
Leasehold improvements | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Leasehold improvements						1,022,000
Mortgage Loans
Class of Stock [Line Items]
Net (losses) gains from fair value changes of mortgage loans			$ (2,861,000)	(4,539,000)	9,111,000
GNMA
Class of Stock [Line Items]
Delinquent GNMA loans that had been previously sold			51,705,000	67,362,000
Residential Mortgage Loans
Class of Stock [Line Items]
Net (losses) gains from fair value changes of mortgage loans			$ 12,259,000	$ 14,732,000	$ 11,706,000
Minimum | Furniture and fixtures
Class of Stock [Line Items]
Property, plant and equipment, useful life			7 years
Minimum | Equipment
Class of Stock [Line Items]
Property, plant and equipment, useful life			3 years
Minimum | Commercial And Commercial Real Estate Loans
Class of Stock [Line Items]
Loans individually evaluated for impairment			$ 250,000
Maximum | Furniture and fixtures
Class of Stock [Line Items]
Property, plant and equipment, useful life			10 years
Maximum | Leasehold improvements
Class of Stock [Line Items]
Property, plant and equipment, useful life			20 years
Maximum | Equipment
Class of Stock [Line Items]
Property, plant and equipment, useful life			7 years
Maximum | Commercial And Commercial Real Estate Loans
Class of Stock [Line Items]
Loans individually evaluated for impairment			$ 250,000
Federal Home Loan Bank of Cincinnati
Class of Stock [Line Items]
Deposits with Federal Home Loan Banks			$ 11,225,000
Lease Agreements | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Finite lived intangible assets						$ 460,000