Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (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
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		$ 153.7				$ 153.7
Other comprehensive income (loss)	$ 7.1	2.2	$ (11.4)	$ 12.2		9.3	$ 0.8
Cumulative effect of change in accounting principle		0.0		2.2	[1]
Ending balance	194.2					194.2
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(572.1)				(572.1)
Other comprehensive income (loss) before reclassifications						(3.0)
Amounts reclassified from accumulated other comprehensive loss to net income						16.4
Other comprehensive income (loss)	5.9		21.0			13.4	31.7
Ending balance	(587.5)					(587.5)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(382.0)				(382.0)
Other comprehensive income (loss) before reclassifications						9.3
Amounts reclassified from accumulated other comprehensive loss to net income						0.0
Other comprehensive income (loss)	9.0		(31.5)			9.3	(31.4)
Ending balance	(372.7)					(372.7)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		0.8				0.8
Other comprehensive income (loss) before reclassifications						(14.5)
Amounts reclassified from accumulated other comprehensive loss to net income						0.7
Other comprehensive income (loss)	(7.9)		0.1			(13.8)	0.4
Ending balance	(13.0)					(13.0)
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(953.3)				(953.3)
Other comprehensive income (loss) before reclassifications						(8.2)
Amounts reclassified from accumulated other comprehensive loss to net income						17.1
Other comprehensive income (loss)	7.0	1.9	$ (10.4)	11.1		8.9	$ 0.7
Cumulative effect of change in accounting principle		(28.8)		$ (1.1)	[1]
Ending balance	$ (973.2)					$ (973.2)
Accounting Standards Update 2018-02 | Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative effect of change in accounting principle		(28.8)
Accounting Standards Update 2018-02 | AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative effect of change in accounting principle		$ (28.8)

[1] Effective January 1, 2018, we adopted the provisions of ASU 2014-09, Revenue From Contracts with Customers, ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, and ASU 2016-16, Intra-Entity Transfers of Assets Other Than Inventory. We recognized a cumulative effect adjustment to January 1, 2018 retained earnings as a result of adopting each of these standards. See Note 1 for further details of the impact of each standard.