Summary of Significant Accounting Policies - Goodwill and Intangible Assets, Net (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) segment reporting_unit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2000 USD ($)
Goodwill [Abstract]
Goodwill	$ 3,076,926,000	$ 3,076,926,000	$ 3,066,700,000
Number of reportable segments | segment		1
Number of reporting units | reporting_unit		1
Intangible Assets, Net [Abstract]
Intangible assets, net of accumulated amortization	55,366,000	$ 55,366,000	79,424,000
Finite-lived intangible assets subject to amortization, net	41,900,000	41,900,000	65,900,000
Indefinite-lived intangible assets not subject to amortization	13,500,000	13,500,000	13,500,000
Gross carrying amount of intangible assets	468,900,000	468,900,000	475,100,000
Accumulated amortization of intangible assets	427,000,000.0	427,000,000.0	409,200,000
Amortization expense on intangible assets		28,759,000	27,781,000	$ 27,896,000
Impairment of intangible assets	3,100,000
Impairment of finite-lived intangible assets		0	0
Future estimated annual amortization expense [Abstract]
2019	21,000,000	21,000,000
2020	5,000,000	5,000,000
2021	4,000,000	4,000,000
2022	4,000,000	4,000,000
2023	4,000,000	$ 4,000,000
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					$ 3,000,000,000.0
Goodwill	2,800,000,000	$ 2,800,000,000
Intangible Assets, Net [Abstract]
Finite-lived intangible assets subject to amortization, net	15,500,000	15,500,000	$ 36,200,000
Various smaller acquisitions
Goodwill [Abstract]
Goodwill	$ 276,000,000	$ 276,000,000