Basis of presentation - Additional Information (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Jul. 19, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 $ / shares shares	Jun. 30, 2019 USD ($) branch $ / shares	Jun. 30, 2018 $ / shares	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Class of Stock [Line Items]
Number of full-service branches | branch				65
Cash dividends declared (USD per share) | $ / shares		$ 0.06	$ 0.08	$ 0.06	$ 0.16
Operating lease right-of-use assets		$ 35,872		$ 35,872			$ 0
Operating lease liabilities		38,722		38,722			0
Cumulative effect of new accounting principal						$ (1,309)		$ 0
Leasehold improvements		92,407		92,407			86,882
Finite lived intangible assets		$ 19,945		$ 19,945			$ 11,628
Accounting Standards Update 2016-02
Class of Stock [Line Items]
Operating lease right-of-use assets						32,545
Operating lease liabilities						34,876
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)		$ (109)
Retained earnings | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Cumulative effect of new accounting principal						(1,309)
Cumulative effect of new accounting principle in period of adoption, net of deferred taxes						461
Subsequent Event
Class of Stock [Line Items]
Cash dividends declared (USD per share) | $ / shares	$ 0.08
Dividends declared	$ 2,555
James W. Ayers
Class of Stock [Line Items]
Percentage of voting power (less than)				50.00%
Lease Agreements | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Finite lived intangible assets						460
Leasehold improvements | Accounting Standards Update 2016-02
Class of Stock [Line Items]
Leasehold improvements						$ 1,022