Basis of presentation - Additional Information (Details)					3 Months Ended		12 Months Ended
	Nov. 14, 2018 branch	May 26, 2017 USD ($) $ / shares shares	Sep. 15, 2016 USD ($) $ / shares shares	Jun. 28, 2016 shares	Mar. 31, 2019	Jun. 30, 2018 shares	Dec. 31, 2018 USD ($) branch Method shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)	Jul. 31, 2016 shares	Jun. 30, 2016 shares
Class of Stock [Line Items]
Number of full-service branches | branch							56
Stock split ratio				100
Common stock, shares issued (in shares) | shares							30,724,532	30,535,517
Common stock, shares authorized (in shares) | shares							75,000,000	75,000,000			75,000,000	25,000,000
Proceeds from issuance of common stock, net of offering costs							$ 1,196,000	$ 153,356,000	$ 116,054,000
Effective tax rate, percent							24.20%	28.70%	34.90%
OTTI on available for sale securities							$ 0	$ 0	$ 0
Other than temporary impairment							0	945,000	0
Transfers from loans to loans held for sale							11,888,000	0	0
Adjustments for transfers to loans HFS							$ 349,000
Percent of remaining principal allowed to buy back under GNMA optional repurchase programs							100.00%
Deposits with Federal Home Loan Banks							$ 5,225,000	0
Loans individually evaluated for impairment							13,716,000	9,373,000	12,951,000
Loans collectively evaluated for impairment							3,584,796,000	3,068,703,000	1,819,775,000
Impairment losses on mortgage servicing rights							0	0	4,678,000
Goodwill impairment loss							0	0	0
Deferred tax liability increased due to change in taxable status							0	0	13,181,000
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
Advertising costs							$ 13,139,000	12,957,000	10,608,000
Adoption of ASU 2016-01 (See Note 1)										$ 0
ASC 860-50-35, Transfers and Servicing
Class of Stock [Line Items]
Adjustment to retained earnings								615,000
ASU 2018-02
Class of Stock [Line Items]
Reclassification from accumulated other comprehensive income to retained earnings										652,000
Premises
Class of Stock [Line Items]
Property, plant and equipment, useful life							40 years
Residential Mortgage Loans
Class of Stock [Line Items]
Residential mortgage loans transferred by bank							$ 14,732,000	11,706,000	17,963,000
Mortgage Loans
Class of Stock [Line Items]
Net (losses) gains from fair value changes of mortgage loans							(4,539,000)	9,111,000	(2,289,000)
GNMA
Class of Stock [Line Items]
Residential mortgage loans transferred by bank							0	43,035,000
Transfers from loans to loans held for sale							0	$ 43,035,000	$ 0
Delinquent GNMA loans that had been previously sold							67,362,000
Pro forma
Class of Stock [Line Items]
Effective tax rate, percent									36.75%
Initial Public Offering
Class of Stock [Line Items]
Common stock sold and issued, shares (in shares) | shares			6,764,704
Common stock sold and issued, price per share (USD per share) | $ / shares			$ 19
Proceeds from issuance of common stock, net of offering costs			$ 115,525,000
Distribution to majority shareholder from net proceeds			55,000,000
Repayments of aggregate principal amount of subordinated notes from net proceeds			$ 10,075,000
Placement agent and other offering costs							$ 671,000
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
Proceeds from issuance of common stock, net of offering costs		$ 152,721,000
Placement agent and other offering costs		$ 5,901,000
Secondary Public Offering
Class of Stock [Line Items]
Common stock sold and issued, shares (in shares) | shares						3,680,000
Minimum
Class of Stock [Line Items]
Common stock, shares issued (in shares) | shares				171,800
Common stock, shares authorized (in shares) | shares				250,000
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
Minimum | Scenario, Forecast | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Increase in assets as a result of new accounting guidance, percent					0.40%
Maximum
Class of Stock [Line Items]
Common stock, shares issued (in shares) | shares				17,180,000
Common stock, shares authorized (in shares) | shares				25,000,000
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 collectively evaluated for impairment							$ 250,000
Maximum | Scenario, Forecast | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Increase in assets as a result of new accounting guidance, percent					0.80%
Retained earnings
Class of Stock [Line Items]
Adoption of ASU 2016-01 (See Note 1)										(109,000)
Retained earnings | Accounting Standards Update 2016-01
Class of Stock [Line Items]
Adoption of ASU 2016-01 (See Note 1)										$ 109,000
Federal Home Loan Bank of Cincinnati
Class of Stock [Line Items]
Deposits with Federal Home Loan Banks							$ 5,225,000
Atlantic Capital Bank | TENNESSEE
Class of Stock [Line Items]
Number of branches acquired | branch	11
Atlantic Capital Bank | GEORGIA
Class of Stock [Line Items]
Number of branches acquired | branch	3