Business and Basis of Presentation - Impact of Adoption of ASU 2014-09 (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fees receivable	$ (1,134,964)		$ (1,176,843)
Prepaid expenses and other current assets	175,204		124,632
Allowance for doubtful accounts	(7,100)		(12,700)
Accounts payable and accrued liabilities	$ 532,538		$ 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