Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Derivative [Line Items]
Notional Value	$ 1,413.0	$ 1,681.9	$ 1,620.7	$ 2,532.1
2012
Derivative [Line Items]
Notional Value	185.0
Weighted Average Receive Rate	6.49%
Weighted Average pay rate	0.58%
2013
Derivative [Line Items]
Notional Value	150.0
Weighted Average Receive Rate	6.34%
Weighted Average pay rate	0.58%
Receive Fixed/Pay Variable
Derivative [Line Items]
Notional Value	$ 335.0
Weighted Average Receive Rate	6.42%
Weighted Average pay rate	0.58%