Summary of Significant Accounting Policies - Schedule of Repurchase Activity (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
AB Holding Units purchased in periods (in units)	5.4	6.0
Dollar amount paid for AB Holding Units acquired	$ 149.0	$ 172.6
Open-market purchases of AB Holding Units (in units)	3.1	2.9
Dollar amount paid for open-market purchases of AB holding units	$ 74.0	$ 82.7