Summary of Significant Accounting Policies - Goodwill and Intangible Assets, Net (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) segment reporting_unit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 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	$ 105,784	134,606
Finite-lived intangible assets subject to amortization, net	92,300	121,100
Indefinite-lived intangible assets not subject to amortization	13,500	13,500
Gross carrying amount of intangible assets	473,700	476,100
Accumulated amortization of intangible assets	381,400	355,000
Amortization expense on intangible assets	27,896	26,311	$ 25,798
Future estimated annual amortization expense [Abstract]
2018	28,000
2019	28,000
2020	20,000
2021	4,000
2022	$ 4,000
Minimum [Member]
Intangible Assets, Net [Abstract]
Intangible assets acquired, estimated useful life	6 years
Maximum [Member]
Intangible Assets, Net [Abstract]
Intangible assets acquired, estimated useful life	20 years
Sanford C Bernstein Inc Acquisition [Member]
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	56,900	$ 77,600
Series of Individually Immaterial Business Acquisitions [Member]
Goodwill [Abstract]
Goodwill	$ 266,000