Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Payment Arrangement [Abstract]
AB Holding Units purchased (in units)		1,200,000	2,000,000	1,200,000
Amount paid for AB Holding Units acquired		$ 32,900	$ 58,600	$ 35,200
Open-market purchases of AB Holding Units (in units)	1,900,000	1,200,000
Amount paid for open-market purchases of AB Holding Units	$ 55,200	$ 32,900
Units granted to employees and Eligible Directors (in units)			1,700,000	2,400,000
Units issued upon exercise of options (in units)			406,235	500,000
Proceeds from stock options exercised			$ 8,951	$ 8,340