Valuation and Qualifying Account - Allowance for Doubtful Accounts (Details) - Allowance for Doubtful Accounts [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves
Balance at Beginning of Period	$ 513	$ 552	$ 725
Credited to Costs and Expenses	150	0	100
Deductions	252	39	273
Balance at End of Period	411	513	552
AllianceBernstein L.P. [Member]
Movement in Valuation Allowances and Reserves
Amounts written-off as uncollectible	$ 252	$ 39	$ 273