Basis of presentation - Additional Information (Details) $ / shares in Units, $ in Thousands		3 Months Ended			9 Months Ended
	Oct. 17, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 shares	Sep. 30, 2019 USD ($) branch $ / shares	Sep. 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					68
Cash dividends declared (USD per share) | $ / shares		$ 0.08	$ 0.06		$ 0.24	$ 0.12
Operating lease right-of-use assets		$ 34,812			$ 34,812			$ 0
Operating lease liabilities		37,760			37,760			0
Cumulative effect of new accounting principal							$ (1,309)		$ 0
Leasehold improvements		91,815			91,815			86,882
Finite lived intangible assets		$ 18,748			$ 18,748			$ 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	$ 2,556
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