Accumulated other comprehensive income (loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			$ 330.6
Other comprehensive income (loss)	$ 4.0	$ 15.8	29.2	$ (27.4)
Ending balance	358.1		358.1
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(570.5)
Other comprehensive income (loss) before reclassifications			(1.4)
Amounts reclassified from accumulated other comprehensive loss			16.2
Other comprehensive income (loss)			14.8
Ending balance	(555.7)		(555.7)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(322.6)
Other comprehensive income (loss) before reclassifications			13.6
Amounts reclassified from accumulated other comprehensive loss			0.0
Other comprehensive income (loss)	(3.5)	6.5	13.6	(42.4)
Ending balance	(309.0)		(309.0)
Unrealized Gains (Losses) on Available-for-Sale Securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			1.1
Other comprehensive income (loss) before reclassifications			0.3
Amounts reclassified from accumulated other comprehensive loss			(0.3)
Other comprehensive income (loss)	(0.2)	(0.1)	0.0	(0.1)
Ending balance	1.1		1.1
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			0.1
Other comprehensive income (loss) before reclassifications			(1.7)
Amounts reclassified from accumulated other comprehensive loss			1.5
Other comprehensive income (loss)	0.0	0.4	(0.2)	0.4
Ending balance	(0.1)		(0.1)
Accumulated Other Comprehensive Loss
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(891.9)
Other comprehensive income (loss) before reclassifications			10.8
Amounts reclassified from accumulated other comprehensive loss			17.4
Other comprehensive income (loss)	3.8	$ 15.3	28.2	$ (26.5)
Ending balance	$ (863.7)		$ (863.7)