Financial instruments and fair value measurements - Marketable Securities and Financial Instruments on Fair Value Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current	$ 0.9	$ 0.9
Noncurrent	256.7	256.8
Currency forward contracts	(0.1)	1.8
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current
Noncurrent	3.6	3.5
Currency forward contracts	(0.1)	1.8
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current	0.9	0.9
Noncurrent	3.1	3.3
Currency forward contracts
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current
Noncurrent	250.0	250.0
Currency forward contracts