Office Facilities - Summary of Office Facilities Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation and amortization of furniture, equipment and leasehold improvements	$ 5,657	$ 6,788	$ 7,188
Office facilities, gross	110,013	111,306
Less accumulated depreciation	(82,826)	(84,073)
Total	27,187	27,233
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Office facilities, gross	53,260	55,210
Leasehold improvements
Property, Plant and Equipment [Line Items]
Office facilities, gross	$ 56,753	$ 56,096