Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Y
Derivative [Line Items]
U.S. Treasury Note, maturity term (in years)	10
Net amount of settlement gains (losses) on derivative instruments included in accumulated OCI to be amortized over next 12 months, before tax	$ (5.7)
Net amount of settlement gains (losses) on derivative instruments included in accumulated OCI to be amortized over next 12 months, after tax	(3.5)
2008 Interest Rate Protection Agreement
Derivative [Line Items]
Proceeds from or (payments to) settlement of interest rate protection agreement	(4.4)
2010 Interest Rate Protection Agreement
Derivative [Line Items]
Proceeds from or (payments to) settlement of interest rate protection agreement	9.9
2011 Interest Rate Protection Agreement
Derivative [Line Items]
Proceeds from or (payments to) settlement of interest rate protection agreement	$ (65.5)