Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
New York City unincorporated business tax rate	4.00%
Income tax expense for repatriation of foreign earnings	$ 22,500,000
Income tax expense to reduce deferred tax assets due to lower corporate tax rates	3,300,000
Interest expense recorded in income tax expense	300,000	$ 700,000	$ 400,000
Accrued interest payable recorded on the consolidated statements of financial condition	700,000	1,700,000	1,000,000
Accrued penalties recorded on the consolidated statements of financial condition	0	0	0
Reductions related to closed years/settlements with tax authorities	2,269,000	0	0
Decrease in accrued interest	$ 400,000
Minimum years that remain open and may be subject to examination vary under local law	1 year
Maximum years that remain open and may be subject to examination vary under local law	7 years
Unrecognized tax benefits	$ 8,478,000	$ 12,596,000	$ 12,004,000	$ 11,311,000
City of New York [Member]
Income Tax Examination [Line Items]
Unrecognized tax benefits	$ 5,100,000