Fair Value and Carrying Value of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Carrying Amount	$ 2,101.8	$ 1,671.2
Estimated Fair Value	2,188.7	1,829.8
Series 2005-1 Notes
Debt Instrument [Line Items]
Carrying Amount	310.3	313.8
Estimated Fair Value	319.2	326.1
Series 2007-1 Notes
Debt Instrument [Line Items]
Carrying Amount	300.0	300.0
Estimated Fair Value	334.7	348.3
2010 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	497.8	497.4
Estimated Fair Value	536.6	562.8
2012 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	496.5	496.2
Estimated Fair Value	497.0	528.8
2008 Term Loan
Debt Instrument [Line Items]
Carrying Amount	0	63.8
Estimated Fair Value	0	63.8
2013 Senior Notes
Debt Instrument [Line Items]
Carrying Amount	497.2
Estimated Fair Value	$ 501.2