Fair Value Measurements - Fair Value of Financial Assets and Financial Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Assets:
Investments	$ 155.2		$ 108.4
Fair Value, Measurements, Recurring [Member]
Assets:
Investments	155.2		108.4
Derivative instruments	0	[1]	0	[1]
Liabilities:
Derivative instruments	(306.4)		(336.1)
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	155.2		106.9
Derivative instruments	0		0
Liabilities:
Derivative instruments	0		0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	0		1.5
Derivative instruments	0	[1]	0	[1]
Liabilities:
Derivative instruments	(306.4)		(336.1)
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	0		0
Derivative instruments	0		0
Liabilities:
Derivative instruments	$ 0		$ 0

[1] Amount rounds to zero