Furniture, equipment and leasehold imporvements - Summary of Office Facilities Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation and amortization of furniture, equipment and leasehold improvements	$ 8,143	$ 7,635	$ 6,871
Office facilities, gross	118,720	126,150
Less accumulated depreciation	(90,958)	(94,773)
Total	27,762	31,377
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Office facilities, gross	62,860	62,444
Leasehold improvements
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 55,860	$ 63,706