Timing of Anticipated Settlements of Interest Rate Swaps Outstanding (Detail) (USD $)	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Notional Amount of Derivatives	$ 1,513,200,000	$ 1,309,500,000	$ 1,413,000,000	$ 1,498,700,000	$ 1,538,700,000	$ 1,681,900,000
2012
Derivative [Line Items]
Notional Amount of Derivatives	50,000,000
Weighted Average Receive Rate	6.50%
Weighted Average pay rate	0.36%
2013
Derivative [Line Items]
Notional Amount of Derivatives	150,000,000
Weighted Average Receive Rate	6.34%
Weighted Average pay rate	0.36%
Receive Fixed/Pay Variable
Derivative [Line Items]
Notional Amount of Derivatives	$ 200,000,000
Weighted Average Receive Rate	6.38%
Weighted Average pay rate	0.36%