Goodwill and other intangibles - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Increase in goodwill	$ 17,504	$ 14,419
Other intangibles, gross carrying amount	35,000	30,500
Accumulated amortization of other intangibles	25,300	20,800
Amortization expense of other intangibles	4,600	3,800
Future amortization expense
Expected amortization expense in 2019	4,300
Expected amortization expense in 2020	3,000
Expected amortization expense in 2021	1,100
Expected amortization expense in 2022	400
Expected amortization expense in 2023	200
Employment and Non-compete Agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangibles acquired	$ 4,500
Employment and Non-compete Agreements | Minimum
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization period	1 year
Employment and Non-compete Agreements | Maximum
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization period	3 years
Acquired Software
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangibles acquired		$ 2,600
Amortization period		5 years
Title
Acquired Finite-Lived Intangible Assets [Line Items]
Increase in goodwill	$ 17,504	$ 14,419