Fair Value and Carrying Value of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Carrying Amount	$ 1,707.5		$ 1,243.8
Estimated Fair Value	1,832.0		1,309.8
Series 2005-1 Notes
Debt Instrument [Line Items]
Carrying Amount	318.4	[1]	311.5	[1]
Estimated Fair Value	315.5		316.5
Series 2007-1 Notes
Debt Instrument [Line Items]
Carrying Amount	300.0		300.0
Estimated Fair Value	335.9		332.7
2010 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	497.4		497.3
Estimated Fair Value	554.6		525.6
2012 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	496.1
Estimated Fair Value	530.4
2008 Term Loan
Debt Instrument [Line Items]
Carrying Amount	95.6		135.0
Estimated Fair Value	$ 95.6		$ 135.0

[1] The carrying amount includes an $18.4 million and $11.5 million fair value adjustment on an interest rate hedge at September 30, 2012 and December 31, 2011, respectively.