Fair Value Measurements - Fair Value of Financial Assets and Financial Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets:
Investments	$ 114.7	$ 114.2
Estimate of Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	114.7	114.2
Derivative instruments	0.1	0	[1]
Liabilities:
Derivative instruments	(237.1)	(306.4)
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	114.7	114.2
Derivative instruments	0	0
Liabilities:
Derivative instruments	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Investments	0	0
Derivative instruments	0.1	0	[1]
Liabilities:
Derivative instruments	(237.1)	(306.4)
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