Fair Value Measurements - Fair Value of Financial Assets and Financial Liabilities (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Assets:
Investments	$ 95.3		$ 91.7
Derivative instruments	0		0	[1]
Liabilities:
Derivative instruments	(130.8)		(165.9)
Level 1 [Member]
Assets:
Derivative instruments	0		0
Liabilities:
Derivative instruments	0		0
Level 2 [Member]
Assets:
Investments	0		0
Derivative instruments	0	[1]	0	[1]
Liabilities:
Derivative instruments	(130.8)		(165.9)
Level 3 [Member]
Assets:
Investments	0		0
Derivative instruments	0		0
Liabilities:
Derivative instruments	$ 0		$ 0

[1] Amount rounds to zero