Outstanding Interest Rate Derivatives Designated as Cash Flow Hedges of Interest Rate Risk (Detail) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2012
Derivative [Line Items]
Increase (Decrease) in Fair Value of Derivative Instruments, Not Designated as Hedging Instruments	$ 9,041,000	$ 33,000
Interest Rate Caps
Derivative [Line Items]
Number of interest rate contracts	18	18
Notional	256,371,000	256,371,000
Interest Rate Swaps
Derivative [Line Items]
Number of interest rate contracts	23	23
Notional	$ 601,800,000	$ 601,800,000

The increase (decrease) during the period, which was recognized in earnings, in the unrealized gains or losses on derivative instruments that are not or are no longer designated as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 18

-Subparagraph a, b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 20

-Section 35

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6439004&loc=d3e65527-113976

Name:	us-gaap_IncreaseDecreaseInFairValueOfUnhedgedDerivativeInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration