Fair Value Measurements - Fair Value of Financial Assets and Financial Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member]		Sep. 30, 2011 Fair Value, Measurements, Recurring [Member]		Sep. 30, 2012 Level 1 [Member] Fair Value, Measurements, Recurring [Member]	Sep. 30, 2011 Level 1 [Member] Fair Value, Measurements, Recurring [Member]	Sep. 30, 2012 Level 2 [Member] Fair Value, Measurements, Recurring [Member]		Sep. 30, 2011 Level 2 [Member] Fair Value, Measurements, Recurring [Member]		Sep. 30, 2012 Level 3 [Member] Fair Value, Measurements, Recurring [Member]	Sep. 30, 2011 Level 3 [Member] Fair Value, Measurements, Recurring [Member]
Assets:
Investments	$ 117.8	$ 108.4	$ 117.8		$ 108.4		$ 117.0	$ 106.9	$ 0.8		$ 1.5
Derivative instruments				[1]		[1]				[1]		[1]
Liabilities:
Derivative instruments			$ (347.0)		$ (336.1)				$ (347.0)		$ (336.1)

[1] Amount rounds to zero