Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Notional Value	$ 1,498.7	$ 1,538.7	$ 1,681.9
Weighted Average Receive Rate	6.26%
Weighted Average pay rate	0.37%
Receive Fixed/Pay Variable
Derivative [Line Items]
Notional Value	385.0
2011
Derivative [Line Items]
Notional Value	50.0
Weighted Average Receive Rate	5.16%
Weighted Average pay rate	0.37%
2012
Derivative [Line Items]
Notional Value	185.0
Weighted Average Receive Rate	6.49%
Weighted Average pay rate	0.37%
2013
Derivative [Line Items]
Notional Value	$ 150.0
Weighted Average Receive Rate	6.34%
Weighted Average pay rate	0.37%