Business Combinations - Summary of Final Purchase Price Allocation (Details) - USD ($) $ in Thousands	Jul. 01, 2018	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill		$ 766,174		$ 809,608
Hermes
Business Acquisition [Line Items]
Cash and Cash Equivalents	$ 175,800
Other Current Assets	53,700
Goodwill	114,100
Intangible Assets	320,000
Other Long-Term Assets	40,100
Less: Long-Term Deferred Tax Liability, net	(28,700)
Less: Liabilities Acquired	(162,300)
Less: Fair Value of Redeemable Noncontrolling Interest in Subsidiary	(169,200)
Total Purchase Price Consideration	343,500
Receivables			$ 31,900
Property and equipment	11,200
Accrued Compensation and Benefits	130,300
Hermes | Investment Advisory Contracts
Business Acquisition [Line Items]
Indefinite-lived intangible assets	198,500
Hermes | Trade Name
Business Acquisition [Line Items]
Indefinite-lived intangible assets	49,900
Hermes | Customer Relationships
Business Acquisition [Line Items]
Customer relationships	$ 71,600
Weighted-average useful life	8 years 6 months