Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative [Line Items]
Notional Value	$ 1,432.8	$ 1,413.0	$ 1,681.9	$ 1,620.7
Forecasted in 2013 [Member]
Derivative [Line Items]
Notional Value	$ 150.0
Weighted Average Receive Rate	6.34%
Weighted Average pay rate	0.31%