Accumulated other comprehensive income (loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI	$ (891.9)
Other comprehensive income (loss)	25.2	$ (43.2)
Ending Balance - AOCI	(867.5)
Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI	(891.9)
Other comprehensive income (loss) before reclassifications	15.6
Amounts reclassified from accumulated other comprehensive loss	8.8
Other comprehensive income (loss)	24.4
Ending Balance - AOCI	(867.5)
Benefit Plan Adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI	(570.5)
Other comprehensive income (loss) before reclassifications	(0.8)
Amounts reclassified from accumulated other comprehensive loss	8.1
Other comprehensive income (loss)	7.3
Ending Balance - AOCI	(563.2)
Foreign Currency Translation Adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance - AOCI	(322.6)
Other comprehensive income (loss) before reclassifications	17.1
Amounts reclassified from accumulated other comprehensive loss	0.0
Other comprehensive income (loss)	17.1
Ending Balance - AOCI	(305.5)
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	1.1
Other comprehensive income (loss) before reclassifications	0.2
Amounts reclassified from accumulated other comprehensive loss	0.0
Other comprehensive income (loss)	0.2
Ending Balance - AOCI	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	0.1
Other comprehensive income (loss) before reclassifications	(0.9)
Amounts reclassified from accumulated other comprehensive loss	0.7
Other comprehensive income (loss)	(0.2)
Ending Balance - AOCI	$ (0.1)