Gains and Losses on Derivatives Designated as Hedging Instruments, Cash Flow Hedging (Detail) (Cash Flow Hedging, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) Recognized in AOCI on Derivative (Effective Portion)		$ (0.1)	$ (0.1)	$ (0.5)
Amount of Gain/(Loss) Reclassified from AOCI into Income (Effective Portion)	(0.6)	(0.9)	(1.9)	(2.4)
Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) Recognized in AOCI on Derivative (Effective Portion)		(0.1)	(0.1)	(0.5)
Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) Reclassified from AOCI into Income (Effective Portion)				(0.2)
Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) Reclassified from AOCI into Income (Effective Portion)	$ (0.6)	$ (0.9)	$ (1.9)	$ (2.2)