Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Notional Value	$ 1,336.1	$ 1,403.7	$ 1,432.8	$ 1,513.2	$ 1,309.5	$ 1,413.0
Forecasted in 2013
Derivative [Line Items]
Notional Value	$ 35.0
Weighted Average Receive Rate	6.82%
Weighted Average pay rate	0.25%