Furniture, equipment and leasehold improvements - Summary of Office Facilities Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 120,821	$ 118,720
Less accumulated depreciation	(92,785)	(90,958)
Total	28,036	27,762
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Office facilities, gross	63,334	62,860
Leasehold improvements
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 57,487	$ 55,860