Income Taxes - Additional Information (Detail) £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 11, 2018 GBP (£)	Apr. 11, 2018 USD ($)
Schedule Of Income Tax [Line Items]
Tax effected net operating losses that expire	$ 642
NOL expiration date	2024
Tax effected NOLs - International	$ 9,336	$ 6,605
Accrued for unrecognized tax benefits and interest/penalties related to a tax position claimed or expected to be claimed on a tax return	32,101	34,876
Unrecognized tax benefits that would impact effective tax rate	32,101
Tax effected capital loss	8,226	794
Reduction in unrecognized tax benefits	4,309
Decrease in unrecognized tax benefits in the next 12 months	4,028
Decrease in accrued interest and penalties over the next 12 months	1,231
International Tax Authority [Member]
Schedule Of Income Tax [Line Items]
Tax effected net operating losses that expire	$ 4,930
Operating loss Expiration Period	expire between the years 2036 and 2039
Other Noncurrent Liabilities [Member]
Schedule Of Income Tax [Line Items]
Accrued for unrecognized tax benefits and interest/penalties related to a tax position claimed or expected to be claimed on a tax return	$ 32,101	$ 34,876
Acquisition Of Etfs Business [Member]
Schedule Of Income Tax [Line Items]
Accrued uncertain tax position liability			£ 22,118	$ 31,317
Interest and penalties			£ 5,065	$ 7,171