Accumulated other comprehensive income (loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			$ (792.0)
Other comprehensive income (loss)	$ 15.8	$ 12.9	(27.4)	$ 15.7
Ending Balance - AOCI	(818.5)		(818.5)
Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			(792.0)
Other comprehensive income (loss) before reclassifications			(44.7)
Amounts reclassified from accumulated other comprehensive loss			18.2
Other comprehensive income (loss)			(26.5)
Ending Balance - AOCI	(818.5)		(818.5)
Benefit Plan Adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			(571.7)
Other comprehensive income (loss) before reclassifications			(4.2)
Amounts reclassified from accumulated other comprehensive loss			19.8
Other comprehensive income (loss)			15.6
Ending Balance - AOCI	(556.1)		(556.1)
Foreign Currency Translation Adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			(222.1)
Other comprehensive income (loss) before reclassifications			(42.4)
Amounts reclassified from accumulated other comprehensive loss			0.0
Other comprehensive income (loss)			(42.4)
Ending Balance - AOCI	(264.5)		(264.5)
Unrealized Gains (Losses) on Available-for-Sale Securities | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			1.4
Other comprehensive income (loss) before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive loss			(0.1)
Other comprehensive income (loss)			(0.1)
Ending Balance - AOCI	1.3		1.3
Gains (Losses) on Cash Flow Hedges | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI			0.4
Other comprehensive income (loss) before reclassifications			1.9
Amounts reclassified from accumulated other comprehensive loss			(1.5)
Other comprehensive income (loss)			0.4
Ending Balance - AOCI	$ 0.8		$ 0.8