Long-term Incentive Compensation Plans - Unit Purchase Activity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
Total amount of AB Holding Units Purchased (in units)	0.3	0.9	2.5	2.9
Total Cash Paid for AB Holding Units Purchased	$ 6.8	$ 25.1	$ 54.4	$ 83.7
Open Market Purchases of AB Holding Units Purchased (in units)	0.2	0.6	2.4	2.5
Total Cash Paid for Open Market Purchases of AB Holding Units	$ 6.7	$ 15.3	$ 51.8	$ 70.6