Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Notional Value	$ 1,444.8	$ 1,432.8	$ 1,364.1	$ 1,413.0
Forecasted in 2013 [Member]
Derivative [Line Items]
Notional Value	$ 110.0
Weighted Average Receive Rate	6.71%
Weighted Average pay rate	0.28%