Financial instruments - Short-term Investments and Financial Instruments on Fair Value Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current	$ 0.9	$ 22.5
Noncurrent	256.8	354.1
Currency forward contracts	1.8	(0.8)
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current		20.9
Noncurrent	3.5	100.3
Currency forward contracts	1.8	(0.8)
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current	0.9	1.6
Noncurrent	3.3	3.8
Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Noncurrent	$ 250.0	$ 250.0