Accumulated other comprehensive (loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			$ 330.6
Other comprehensive income (loss)	$ 5.0	$ (31.6)	34.2	$ (59.0)
Ending balance	392.0		392.0
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(570.5)
Other comprehensive income (loss) before reclassifications			(2.0)
Amounts reclassified from accumulated other comprehensive loss			24.6
Other comprehensive income (loss)			22.6
Ending balance	(547.9)		(547.9)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(322.6)
Other comprehensive income (loss) before reclassifications			10.2
Amounts reclassified from accumulated other comprehensive loss			0.0
Other comprehensive income (loss)	(3.4)	(37.3)	10.2	(79.7)
Ending balance	(312.4)		(312.4)
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)	0.0	(0.3)
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			(1.6)
Amounts reclassified from accumulated other comprehensive loss			1.5
Other comprehensive income (loss)	0.1	(0.4)	(0.1)	0.0
Ending balance	0.0		0.0
Accumulated Other Comprehensive Loss
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(891.9)
Other comprehensive income (loss) before reclassifications			6.9
Amounts reclassified from accumulated other comprehensive loss			25.8
Other comprehensive income (loss)	4.5	$ (29.3)	32.7	$ (55.8)
Ending balance	$ (859.2)		$ (859.2)