Investment in AB (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Change in Equity Method Investment [Roll Forward]
Beginning balance			$ 1,605,941
Equity in net income attributable to AB Unitholders	$ 95,921	$ 73,874	282,235	$ 206,989
Changes in accumulated other comprehensive income	(3,014)	$ 6,061	(5,005)	1,426
Cash distributions received from AB			(290,773)	(225,802)
Additional investments with proceeds from exercise of compensatory options to buy AB Holding Units			3,402	147
Capital contributions (from) AB			(1,914)	$ (865)
AB Holding Units retired			(124,923)
AB Holding Units issued to fund long-term incentive compensation plans			108,512
Change in AB Holding Units held by AB for long-term incentive compensation plans			(27,288)
Ending balance	$ 1,550,187		$ 1,550,187