Related Party Transactions (Details) - USD ($)				1 Months Ended		12 Months Ended			36 Months Ended
	Oct. 04, 2016	Oct. 03, 2016	Aug. 13, 2015	Nov. 30, 2015	Aug. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Related Party Transaction [Line Items]
Auction commission revenues						$ 5,200,000	$ 4,100,000	$ 2,700,000
Related party client payable						996,197,000	511,876,000		$ 996,197,000
Accounts Receivable (net)						$ 0	$ 1,800,000		$ 0
Treasury stock acquired (in shares)		2,050,000	2,667,378	1,038,280	3,705,658	961,000	13,144,000	3,706,000	17,811,000
Aggregate purchase price			$ 100,000,000			$ 44,495,000	$ 359,885,000	$ 125,000,000	$ 529,380,000
Cost per share of stock acquired (usd per share)		$ 36.00	$ 37.49		$ 33.73	$ 46.32	$ 27.38	$ 33.73	$ 29.72
Ownership percentage in Sotheby's outstanding Common Stock prior to sale		8.50%
Marcato Capital Management | Sale of stock
Related Party Transaction [Line Items]
Treasury stock acquired (in shares)	2,050,000
Aggregate purchase price	$ 73,800,000
Cost per share of stock acquired (usd per share)	$ 36.00
Ownership percentage in Sotheby's outstanding Common Stock prior to sale		8.50%
Related party consignors
Related Party Transaction [Line Items]
Related party client payable						$ 400,000	$ 3,600,000		$ 400,000