Accumulated other comprehensive loss - Schedule of Changes in Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Comprehensive Income Loss [Line Items]
Marketable securities, Balance at beginning of year	$ 2.1	$ 6.4
Marketable securities, Unrealized gain (loss) arising during the year	(0.2)	(5.2)
Marketable securities, Balance at end of year	1.9	1.2
Currency translation adjustment, Balance at beginning of year	53.3	37.5
Other comprehensive income (loss)	(21.5)	14.1
Currency translation adjustment, Balance at end of year	31.8	51.6
Balance at beginning of period	(42.0)	(23.3)
Other comprehensive income (loss)	(19.2)	8.9
Balance at end of period	(61.9)	(13.2)
Defined Benefit Pension Plans [Member]
Accumulated Comprehensive Income Loss [Line Items]
Balance at beginning of year	(101.5)	(72.6)
Amortization of prior service cost and net losses included in net periodic pension cost	2.1	1.5
Balance at end of year	(99.4)	(71.1)
OPEB [Member]
Accumulated Comprehensive Income Loss [Line Items]
Balance at beginning of year	4.1	5.4
Amortization of prior service credit	(0.3)	(0.3)
Balance at end of year	$ 3.8	$ 5.1