Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 1,096.1	$ 1,106.5	$ 2,160.8	$ 1,618.9
Total service revenue
Disaggregation of Revenue [Line Items]
Revenue	859.1	806.1	1,708.2	1,149.6
Commissions
Disaggregation of Revenue [Line Items]
Revenue	236.3	233.3	463.7	363.8
Buyer fees
Disaggregation of Revenue [Line Items]
Revenue	485.2	465.1	987.9	604.6
Marketplace services revenue
Disaggregation of Revenue [Line Items]
Revenue	137.6	107.7	256.6	181.2
Inventory sales revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 237.0	$ 300.4	$ 452.6	$ 469.3