Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Notional Value	$ 1,364.1	$ 1,413.0	$ 1,615.9	$ 1,681.9
2012
Derivative [Line Items]
Notional Value	140.0
Weighted Average Receive Rate	6.49%
Weighted Average pay rate	0.47%
2013
Derivative [Line Items]
Notional Value	150.0
Weighted Average Receive Rate	6.34%
Weighted Average pay rate	0.47%
Receive Fixed/Pay Variable
Derivative [Line Items]
Notional Value	$ 290.0
Weighted Average Receive Rate	6.41%
Weighted Average pay rate	0.47%