Income taxes - Component of Income Taxes Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Income Tax Expenses [Line Items]
Expected tax expense (benefit), at U.S. federal statutory income tax rate of 35%	$ (24.5)	$ 62.2
Incremental tax on earnings of non-U.S. companies	(1.4)	4.0
Non-U.S. tax rates	(0.4)	(7.7)
Adjustment to the reserve for uncertain tax positions, net	2.0	0.5
Nondeductible expenses	2.5	0.6
U.S. state income taxes and other, net		(0.6)
Income tax expense (benefit)	(21.8)	59.0
Comprehensive provision for income taxes (benefit) allocable to:
Continuing operations	(21.8)	59.0
Discontinued operations		0.6
Other comprehensive income (loss):
Marketable securities	3.6	(5.6)
Currency translation	(3.8)
Total	(21.0)	54.6
Defined Benefit Pension Plans [Member]
Other comprehensive income (loss):
Defined benefit plans	1.2	0.8
OPEB [Member]
Other comprehensive income (loss):
Defined benefit plans	$ (0.2)	$ (0.2)