Long-term Incentive Compensation Plans - Unit Purchase Activity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Payment Arrangement [Abstract]
Total amount of AB Holding Units Purchased (in units)	0.9	1.3	1.9	2.2
Total Cash Paid for AB Holding Units Purchased	$ 38.3	$ 27.8	$ 75.7	$ 47.6
Open Market Purchases of AB Holding Units Purchased (in units)	0.8	1.3	1.4	2.2
Total Cash Paid for Open Market Purchases of AB Holding Units	$ 34.6	$ 27.8	$ 58.8	$ 45.1