Outstanding Interest Rate Derivatives Designated as Cash Flow Hedges of Interest Rate Risk (Detail) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Derivative [Line Items]
Derivative, Cap Interest Rate	600.00%	600.00%
Increase (Decrease) in Fair Value of Derivative Instruments, Not Designated as Hedging Instruments	$ 25,000	$ 58,000
Interest Rate Caps
Derivative [Line Items]
Number of interest rate contracts	18	18
Notional	256,400,000	256,400,000
Interest Rate Swaps
Derivative [Line Items]
Number of interest rate contracts	22	22
Notional	$ 576,800,000	$ 576,800,000