Receivables (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2014 loan	Dec. 31, 2013 loan	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consignor Payment Term		35 days
Buyer receivables classified within other long term assets		$ 22,700,000
Net accounts receivable		913,743,000	812,582,000
Number of types of secured loans		2
Number of Years for Consignor Advance Sale		1 year
Term loan, maturity		2 years
Secured loan issued to refinance auction or private sale receivable		65,600,000	62,200,000
Collections of Notes Receivable Originally Issued to Finance Auction or Private Sale Receivables		38,100,000	76,400,000
Balance of secured loan to refinance auction or private sale receivable		90,400,000	62,900,000
Loans past due		22,409,000	24,129,000
Loans more than 90 days past due accruing interest		0	1,266,000
Non-accrual loans		0	0
Impaired loan		0	0
Unfunded lending commitment to extend additional credit		13,700,000
Number of Significant Unsecured Loans Used to Finance the Purchase of Works of Art Through Private Sales			1,000
Secured loans		644,441,000	474,433,000
Sales Price of Disposal of Equity Method Investment	4,300,000
Equity Method Investment, Realized Gain (Loss) on Disposal	300,000
Proceeds from the sale of equity method investment		300,000	1,225,000	0
Accounts Receivable, Related Parties, Current		1,100,000	2,700,000
Principal [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Secured loans		2,812,000	5,825,000
Other Segments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loan		2,725,000	3,025,000
Finance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan-to-value ratio		48.00%	40.00%
Low auction estimate of collateral		1,349,094,000	1,180,406,000
Non-accrual loans		0	0
Impaired loan		0	0
Secured loans		644,441,000	474,433,000
Agency [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Low auction estimate of collateral		3,900,000
Auction guarantee advances outstanding		25,000,000	28,000,000
Unsecured loan		24,760,000	2,142,000
Accounts Rec, Consignor Paid [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net accounts receivable		116,000,000	130,500,000
Transfer Of Possession Without Payment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net accounts receivable		96,500,000	92,100,000
Notes receivable past due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan-to-value ratio		27.00%
Low auction estimate of collateral		83,500,000
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Buyer payment term		30 days
Loan-to-value ratio		50.00%
Unsecured Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Sales Price of Disposal of Equity Method Investment	3,500,000
Up-front Payment Arrangement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from the sale of equity method investment	800,000
Non Accrual Loans [Member] | Agency [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loan		$ 2,100,000