Furniture, equipment and leasehold imporvements - Summary of Office Facilities Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation and amortization of furniture, equipment and leasehold improvements	$ 7,635	$ 6,871	$ 5,657
Office facilities, gross	126,150	118,170
Less accumulated depreciation	(94,773)	(89,182)
Total	31,377	28,988
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Office facilities, gross	62,444	57,482
Leasehold improvements
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 63,706	$ 60,688