Stockholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ (1,378.6)	$ (1,452.4)	$ (663.9)
Other comprehensive income before reclassifications	(64.9)	346.2	(638.8)
Amounts reclassified from accumulated other comprehensive income	(142.6)	(178.1)	(141.2)
Current period other comprehensive income	(207.5)	168.1	(780.0)
Ending Balance	(1,647.4)	(1,378.6)	(1,452.4)
Accumulated Other Comprehensive Loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,945.3)	(4,113.4)	(3,333.4)
Ending Balance	(4,152.8)	(3,945.3)	(4,113.4)
Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(833.8)	(737.8)	(676.8)
Other comprehensive income before reclassifications	(93.3)	(96.0)	(61.0)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0	0.0
Current period other comprehensive income	(93.3)	(96.0)	(61.0)
Ending Balance	(927.1)	(833.8)	(737.8)
Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,111.5)	(3,375.6)	(2,656.6)
Other comprehensive income before reclassifications	28.4	442.2	(577.8)
Amounts reclassified from accumulated other comprehensive income	(142.6)	(178.1)	(141.2)
Current period other comprehensive income	(114.2)	264.1	(719.0)
Ending Balance	$ (3,225.7)	$ (3,111.5)	$ (3,375.6)