Business and Significant Accounting Policies - Impact of Adoption of ASU 2014-09 (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fees receivable	$ (1,255,118)		$ (1,176,843)
Prepaid expenses and other current assets	165,237		124,632
Allowance for doubtful accounts	(7,700)		(12,700)
Accounts payable and accrued liabilities	$ 710,113		$ 666,821
Accounting Standards Update 2014-09 \| Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fees receivable		$ 26,700
Prepaid expenses and other current assets		26,700
Allowance for doubtful accounts		6,200
Accounts payable and accrued liabilities		$ 6,200

Sum of the carrying values as of the balance sheet date of obligations incurred through that date and due within one year (or the operating cycle, if longer), including liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received, taxes, interest, rent and utilities, accrued salaries and bonuses, payroll taxes and fringe benefits.

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Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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Amount of asset related to consideration paid in advance for costs that provide economic benefits in future periods, and amount of other assets that are expected to be realized or consumed within one year or the normal operating cycle, if longer.

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