Segment Reporting (Schedule of Auction Segment Revenues) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Auction commissions									$ 758,213	$ 687,853	$ 622,391
Private sale commissions									60,183	88,711	74,632
Auction guarantee and inventory activities									(15,462)	(2,186)	(1,623)
Other Agency revenues									22,192	19,801	21,831
Total Agency segment revenues	351,224	94,201	335,817	156,811	339,201	107,864	304,868	101,745	938,053	853,678	768,492
Intercompany private sale commission revenues charged by Agency to Principal	309,582	76,229	316,187	123,128	327,592	76,929	294,943	94,175	825,126	793,639	717,231
Agency [Member]
Segment Reporting Information [Line Items]
Total Agency segment revenues									825,126	794,179	717,231
Reconciling Items [Member]
Segment Reporting Information [Line Items]
Total Agency segment revenues									(9,005)	(10,491)	(7,779)
Intercompany private sale commission revenues charged by Agency to Principal									$ 0	$ (540)	$ 0