Aggregate Notional Amount and Estimated Net Fair Value of Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Notional amount	$ 1,123,770	$ 1,146,720
Fair Value amount	(121,930)	(150,305)
Interest rate swaps
Derivative [Line Items]
Notional amount	1,087,770	1,110,720
Fair Value amount	(122,936)	(151,358)
Interest rate caps
Derivative [Line Items]
Notional amount	36,000	36,000
Fair Value amount	$ 1,006	$ 1,053