Summary of Significant Accounting Policies - Goodwill and Intangible Assets, Net (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) segment reporting_unit	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2000 USD ($)
Goodwill [Abstract]
Goodwill	$ 3,066,700	$ 3,044,807
Number of reportable segments | segment	1
Number of reporting units | reporting_unit	1
Intangible Assets, Net [Abstract]
Intangible assets, net of accumulated amortization	$ 134,606	145,710
Definite-lived intangible assets subject to amortization, net	121,100	132,200
Indefinite-lived intangible assets not subject to amortization	13,500	13,500
Gross carrying amount of intangible assets	476,100	460,800
Accumulated amortization of intangible assets	355,000	328,600
Amortization expense on intangible assets	26,311	25,798	$ 24,916
Future estimated annual amortization expense [Abstract]
2017	28,000
2018	28,000
2019	28,000
2020	20,000
2021	$ 5,000
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]
Definite-lived intangible assets subject to amortization, net	77,600	$ 98,300
Series of Individually Immaterial Business Acquisitions [Member]
Goodwill [Abstract]
Goodwill	$ 266,000