Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
AB Holding Units purchased (in units)	2,000,000	100,000
Amount paid for AB Holding Units acquired	$ 58,600	$ 2,300
Open-market purchases of AB Holding Units (in units)	1,900,000
Amount paid for open-market purchases of AB Holding Units	$ 55,200
Units granted to employees and Eligible Directors (in units)	100,000	700,000
Units issued upon exercise of options (in units)	334,616	200,000
Proceeds from stock options exercised	$ 7,382	$ 4,009