Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Payment Arrangement [Abstract]
Holding units purchased in period (in units)		1.2	2.0	1.2
Dollar amount paid for holding units acquired		$ 32,900	$ 58,600	$ 35,200
Open-market purchases of holding units (in units)	1.9	1.2
Dollar amount paid for open-market purchases of holding units	$ 55,200	$ 32,900
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued upon options exercised (in units)			0.4	0.5
Proceeds from stock options exercised			$ 8,951	$ 8,314
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			1.7	2.4