Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Holding units purchased in period (in units)	2.0	0.1
Dollar amount paid for holding units acquired	$ 58,600	$ 2,300
Open-market purchases of holding units (in units)	1.9
Dollar amount paid for open-market purchases of holding units	$ 55,200
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued upon options exercised (in units)	0.3	0.2
Proceeds from stock options exercised	$ 7,382	$ 4,008
Employees and Eligible Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted AB Holding Unit awards granted to employees and Eligible Directors	0.1	0.7