Investment in AB (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in Equity Method Investment
Investment in AB as of January 1,				$ 1,554,203				$ 1,490,701	$ 1,554,203	$ 1,490,701
Equity in net income attributable to AB Unitholders	$ 101,415	$ 73,874	$ 63,201	$ 69,914	$ 87,909	$ 66,722	$ 59,023	$ 52,638	308,404	266,292	$ 270,647
Changes in accumulated other comprehensive income (loss)									6,730	(1,167)	(5,998)
Cash distributions received from AB									(298,919)	(249,463)	(308,042)
Additional investments with proceeds from exercises of compensatory options to buy AB Holding Units									147	11,511	16,589
Capital contributions to AB										270
Capital contributions from AB									(867)		(1,440)
AB Holding Units retired									(78,388)	(110,752)	(194,544)
AB Holding Units issued to fund long-term incentive compensation plans									107,366	146,488	168,955
Change in AB Holding Units held by AB for long-term incentive compensation plans									7,265	323
Investment in AB as of December 31,	$ 1,605,941				$ 1,554,203				$ 1,605,941	$ 1,554,203	$ 1,490,701