Accumulated Other Comprehensive Loss - Changes in accumulated other comprehensive loss (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Changes in accumulated other comprehensive loss
Beginning balance	$ 2,565.1	$ 2,562.0	$ 4,834.6	$ 4,858.0	$ 2,562.0	$ 4,858.0
Other comprehensive loss, net of tax, before reclassifications					(13.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(3.5)
Less: noncontrolling interest	6.2	3.8	0.5	(1.9)	10.0	(1.4)
Other comprehensive income (loss)					(10.6)
Ending balance	2,529.6	2,565.1	4,671.8	4,834.6	2,529.6	4,671.8
Amounts reclassified from accumulated other comprehensive loss, tax expense (benefit)					(1.0)
Currency translation adjustments
Changes in accumulated other comprehensive loss
Beginning balance		(1.3)			(1.3)
Other comprehensive loss, net of tax, before reclassifications					0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					0.0
Other comprehensive income (loss)					0.0
Ending balance	(1.3)				(1.3)
Amounts reclassified from accumulated other comprehensive loss, tax expense (benefit)					0.0
Unrealized gain/(loss) on derivative financial instruments
Changes in accumulated other comprehensive loss
Beginning balance		(8.3)			(8.3)
Other comprehensive loss, net of tax, before reclassifications					(13.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(1.9)
Other comprehensive income (loss)					(12.2)
Ending balance	(20.5)				(20.5)
Amounts reclassified from accumulated other comprehensive loss, tax expense (benefit)					(0.4)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Noncontrolling Interest [Member]
Changes in accumulated other comprehensive loss
Less: noncontrolling interest					(0.6)
Net actuarial gains/(losses) of defined benefit plans
Changes in accumulated other comprehensive loss
Beginning balance		(107.2)			(107.2)
Other comprehensive loss, net of tax, before reclassifications					0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(1.6)
Other comprehensive income (loss)					1.6
Ending balance	(105.6)				(105.6)
Amounts reclassified from accumulated other comprehensive loss, tax expense (benefit)					(0.6)
Accumulated Other Comprehensive Loss, noncontrolling interest
Changes in accumulated other comprehensive loss
Beginning balance	350.6	351.2	352.9	356.9	351.2	356.9
Less: noncontrolling interest	(0.3)	(0.3)	(0.4)	(0.4)	(0.6)
Ending balance	350.8	350.6	350.2	352.9	350.8	350.2
Accumulated Other Comprehensive Loss
Changes in accumulated other comprehensive loss
Beginning balance	(120.9)	(116.8)	(122.0)	(104.8)	(116.8)	(104.8)
Less: noncontrolling interest	6.5	4.1	0.9	(1.5)
Ending balance	$ (127.4)	$ (120.9)	$ (122.9)	$ (122.0)	$ (127.4)	$ (122.9)