Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in AOCI [Roll Forward]
BALANCE	$ 1,215.9	$ 1,164.1	$ 1,255.7	$ 1,095.7
Other comprehensive income (loss) before reclassifications:
Pre-tax			(75.6)	(37.1)
Tax effect			10.9	6.0
Amounts reclassified from accumulated other comprehensive income (loss):
Provision (benefit) for income taxes			(1.7)	0.2
Net current period other comprehensive income (loss)	7.0	(17.0)	(59.3)	(31.7)
BALANCE	1,356.9	1,186.3	1,356.9	1,186.3
Floorplan interest expense (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			5.3	(0.4)
Other interest expense, net (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			1.7	(0.4)
Realized (gain) loss on interest rate swap termination (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.1	(0.1)
Accumulated other comprehensive income (loss)
Increase (Decrease) in AOCI [Roll Forward]
BALANCE	(213.3)	(152.5)	(147.0)	(137.8)
Amounts reclassified from accumulated other comprehensive income (loss):
Net current period other comprehensive income (loss)	7.0	(17.0)	(59.3)	(31.7)
BALANCE	(206.3)	(169.5)	(206.3)	(169.5)
Accumulated income (loss) on foreign currency translation
Increase (Decrease) in AOCI [Roll Forward]
BALANCE			(142.9)	(146.7)
Other comprehensive income (loss) before reclassifications:
Pre-tax			(24.4)	(12.0)
Tax effect			0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss):
Provision (benefit) for income taxes			0.0	0.0
Net current period other comprehensive income (loss)			(24.4)	(12.0)
BALANCE	(167.2)	(158.7)	(167.2)	(158.7)
Accumulated income (loss) on foreign currency translation | Floorplan interest expense (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0	0.0
Accumulated income (loss) on foreign currency translation | Other interest expense, net (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0	0.0
Accumulated income (loss) on foreign currency translation | Realized (gain) loss on interest rate swap termination (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			0.0	0.0
Accumulated income (loss) on interest rate swaps
Increase (Decrease) in AOCI [Roll Forward]
BALANCE			(4.1)	8.9
Other comprehensive income (loss) before reclassifications:
Pre-tax			(51.3)	(25.2)
Tax effect			10.9	6.0
Amounts reclassified from accumulated other comprehensive income (loss):
Provision (benefit) for income taxes			(1.7)	0.2
Net current period other comprehensive income (loss)			(35.0)	(19.7)
BALANCE	$ (39.1)	$ (10.8)	(39.1)	(10.8)
Accumulated income (loss) on interest rate swaps | Floorplan interest expense (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			5.3	(0.4)
Accumulated income (loss) on interest rate swaps | Other interest expense, net (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			1.7	$ (0.4)
Accumulated income (loss) on interest rate swaps | Realized (gain) loss on interest rate swap termination (pre-tax)
Amounts reclassified from accumulated other comprehensive income (loss):
Pre-tax			$ 0.1