Summary of Significant Accounting Policies - Goodwill and Intangible Assets, Net (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) segment reporting_unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2000 USD ($)
Goodwill [Abstract]
Goodwill	$ 3,066,700	$ 3,066,700
Number of reportable segments | segment	1
Number of reporting units | reporting_unit	1
Intangible Assets, Net [Abstract]
Intangible assets, net of accumulated amortization	$ 79,424	105,784
Finite-lived intangible assets subject to amortization, net	65,900	92,300
Indefinite-lived intangible assets not subject to amortization	13,500	13,500
Gross carrying amount of intangible assets	475,100	473,700
Accumulated amortization of intangible assets	409,200	381,400
Amortization expense on intangible assets	27,781	27,896	$ 26,311
Future estimated annual amortization expense [Abstract]
2019	28,000
2020	21,000
2021	5,000
2022	4,000
2023	$ 4,000
Minimum
Intangible Assets, Net [Abstract]
Intangible assets acquired, estimated useful life	6 years
Maximum
Intangible Assets, Net [Abstract]
Intangible assets acquired, estimated useful life	20 years
Sanford C Bernstein Inc Acquisition
Goodwill [Abstract]
Goodwill acquired during period				$ 3,000,000
Goodwill	$ 2,800,000
Intangible Assets, Net [Abstract]
Finite-lived intangible assets subject to amortization, net	36,200	$ 56,900
Various smaller acquisitions
Goodwill [Abstract]
Goodwill	$ 266,000