Office Facilities - Summary of Office Facilities Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization of furniture, equipment and leasehold improvements	$ 6,871	$ 5,657	$ 6,788
Office facilities, gross	118,170	110,013
Less accumulated depreciation	(89,182)	(82,826)
Total	28,988	27,187
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Office facilities, gross	57,482	53,260
Leasehold improvements
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 60,688	$ 56,753