Impact on Interest Expense of Interest Rate Agreements Accounted for as Cash Flow Hedges (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivative [Line Items]
Net interest expense on cash flow interest rate agreements	$ (30,655)	$ (21,972)
Net Interest Income	28,085	32,361
Cash Flow Hedging | Interest Rate Contract
Derivative [Line Items]
Net interest expense on cash flow interest rate agreements	(1,516)	(1,538)
Realized net expense due to net ineffective portion of hedges	(39)	(1)
Realized net losses reclassified from other comprehensive income	(1,036)	(1,064)
Net Interest Income	$ (2,591)	$ (2,603)