Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accounts receivable, principally due to the allowance for doubtful accounts	$ 6,752	$ 7,148
Accrued expenses not currently deductible for tax	442	2,647
Stock based compensation	37,274	41,415
Income tax credits	376	376
Net operating loss carry forwards	41,168	45,969
Investments	37,804	53,379
Accrued escheat	4,768	7,290
Fixed assets, intangibles and other	12,604	15,622
Deferred tax assets before valuation allowance	141,188	173,846
Valuation allowance	(59,349)	(76,395)
Deferred tax assets, net	81,839	97,451
Deferred tax liabilities:
Intangibles—including goodwill	(508,958)	(687,443)
Basis difference in investment in foreign subsidiaries	(39,287)	(48,354)
Prepaid expenses	(1,605)	(3,644)
Property and equipment, principally due to differences between book and tax depreciation, and other	(49,100)	(24,157)
Deferred tax liabilities	(598,950)	(763,598)
Net deferred tax liabilities	$ (517,111)	$ (666,147)