Long-term Incentive Compensation Plans - Unit Purchase Activity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Payment Arrangement [Abstract]
Total amount of AB Holding Units Purchased (in units)	1.3	0.0	2.2	2.0
Total Cash Paid for AB Holding Units Purchased	$ 27.8	$ 0.0	$ 47.6	$ 58.6
Open Market Purchases of AB Holding Units Purchased (in units)	1.3	0.0	2.2	1.9
Total Cash Paid for Open Market Purchases of AB Holding Units	$ 27.8	$ 0.0	$ 45.1	$ 55.2