Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2021		Mar. 31, 2020	Dec. 31, 2020
Schedule Of Income Tax [Line Items]
Tax effected NOLs - US	$ 382
NOL expiration date	2024
Tax effected NOLs - International	$ 2,084			$ 2,167
Federal statutory rate	21.00%		21.00%
Accrued for unrecognized tax benefits and interest/penalties related to a tax position claimed or expected to be claimed on a tax return	$ 22,222			27,016
Tax effected capital loss	$ 16,596			16,596
Estimated effective rate	(14.90%)		21.50%
Income tax expense/(benefit)	$ (1,969)		$ (2,371)
Reduction in unrecognized tax benefits	5,171	[1]	5,981
Non-taxable gain recognized upon sale of our Canadian ETF business			$ 2,877
Decrease in unrecognized tax benefits in next 12 months, including interest and penalties	7,067
Decrease in accrued interest and penalties over the next 12 months	2,016
Unremitted earnings – International subsidiaries	97			$ 138
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	$ 22,222

[1] Recorded as an income tax benefit of $5,171 during the three months ended March 31, 2021, along with an equal and offsetting amount recorded in other losses, net, to recognize a reduction in the indemnification asset. During the three months ended March 31, 2020, an income tax benefit of $5,981 was recorded along with an equal and offsetting amount in other losses, net.