Accumulated Other Comprehensive Loss - Changes in accumulated other comprehensive loss (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Changes in accumulated other comprehensive loss
Beginning balance		$ 4,858.0
Other comprehensive income (loss), net of tax, before reclassifications		0.1
Amounts reclassified from accumulated other comprehensive loss, net of tax expense (benefit) of $-, $0.3, $(0.4), and $(0.1)		1.8
Less: noncontrolling interest		(1.9)	$ (2.1)
Other comprehensive income (loss)		1.5
Ending balance	[1]	4,830.6
Amounts reclassified from accumulated other comprehensive loss, tax benefit		0.1
Currency translation adjustments
Changes in accumulated other comprehensive loss
Beginning balance		(22.4)
Other comprehensive income (loss), net of tax, before reclassifications		(0.7)
Amounts reclassified from accumulated other comprehensive loss, net of tax expense (benefit) of $-, $0.3, $(0.4), and $(0.1)		0.0
Other comprehensive income (loss)		(0.7)
Cumulative effect of new accounting principle		(0.2)
Ending balance		(23.3)
Amounts reclassified from accumulated other comprehensive loss, tax benefit		0.0
Currency translation adjustments, noncontrolling interest
Changes in accumulated other comprehensive loss
Less: noncontrolling interest		0.0
Unrealized gain/(loss) on derivative financial instruments
Changes in accumulated other comprehensive loss
Beginning balance		0.3
Other comprehensive income (loss), net of tax, before reclassifications		0.8
Amounts reclassified from accumulated other comprehensive loss, net of tax expense (benefit) of $-, $0.3, $(0.4), and $(0.1)		1.0
Other comprehensive income (loss)		1.4
Cumulative effect of new accounting principle		0.1
Ending balance		1.8
Amounts reclassified from accumulated other comprehensive loss, tax benefit		(0.3)
Unrealized loss on derivative financial instruments, noncontrolling interest
Changes in accumulated other comprehensive loss
Less: noncontrolling interest		(0.4)
Net actuarial gains/(losses) of defined benefit plans
Changes in accumulated other comprehensive loss
Beginning balance		(82.7)
Other comprehensive income (loss), net of tax, before reclassifications		0.0
Amounts reclassified from accumulated other comprehensive loss, net of tax expense (benefit) of $-, $0.3, $(0.4), and $(0.1)		0.8
Other comprehensive income (loss)		0.8
Cumulative effect of new accounting principle		(18.6)
Ending balance		(100.5)
Amounts reclassified from accumulated other comprehensive loss, tax benefit		0.4
Net actuarial gains/(losses) of defined benefit plans, noncontrolling interest
Changes in accumulated other comprehensive loss
Less: noncontrolling interest		0.0
Accumulated Other Comprehensive Loss
Changes in accumulated other comprehensive loss
Beginning balance		(104.8)
Less: noncontrolling interest		(1.5)
Cumulative effect of new accounting principle		(18.7)
Ending balance		(122.0)
Accumulated Other Comprehensive Loss, noncontrolling interest
Changes in accumulated other comprehensive loss
Beginning balance		356.9
Less: noncontrolling interest		(0.4)
Ending balance		352.9
Cost of revenues
Changes in accumulated other comprehensive loss
Before-tax reclassification of net actuarial gains/(losses) of defined benefit plans included in cost of revenues		$ 0.8

[1] (unaudited)