Fair Value and Carrying Value of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Carrying Amount	$ 1,671.2		$ 1,243.8
Estimated Fair Value	1,829.8		1,318.3
Series 2005-1 Notes
Debt Instrument [Line Items]
Carrying Amount	313.8	[1]	311.5	[1]
Estimated Fair Value	326.1	[1]	316.5	[1]
Series 2007-1 Notes
Debt Instrument [Line Items]
Carrying Amount	300.0		300.0
Estimated Fair Value	348.3		332.7
2010 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	497.4		497.3
Estimated Fair Value	562.8		534.1
2012 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	496.2
Estimated Fair Value	528.8
2008 Term Loan
Debt Instrument [Line Items]
Carrying Amount	63.8		135.0
Estimated Fair Value	$ 63.8		$ 135.0

[1] The carrying amount includes a $13.8 million and $11.5 million fair value adjustment on an interest rate hedge at December 31, 2012 and 2011, respectively.