Derivative Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivative [Line Items]
Lender accelerated indebtedness repayment threshold amount	$ 75,000,000
Fair value of effective cash flow hedges	8,700,000		5,500,000
Ineffective portion of cash flow hedges	0		0		0
Reclassified as increase to interest expense	6,100,000
Notional Amount	706,671,000		246,973,000
Fair Value at Significant Other Observable Inputs (Level 2)	(8,675,000)		(5,484,000)
Termination date	January 2012
Swap 1 [Member]
Derivative [Line Items]
Notional Amount	69,612,000	[1]	66,563,000	[1]
Type of Derivative	Swap
Strike Rate	2.98%
Effective Date	Apr. 06, 2009
Expiration Date	Nov. 30, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(1,552,000)		(2,363,000)
Swap 2 [Member]
Derivative [Line Items]
Notional Amount	13,335,000	[2]	13,319,000	[2]
Type of Derivative	Swap
Strike Rate	3.981%
Effective Date	May 17, 2006
Expiration Date	Jul. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(275,000)		(583,000)
Swap 3 [Member]
Derivative [Line Items]
Notional Amount	9,649,000	[2]	9,636,000	[2]
Type of Derivative	Swap
Strike Rate	4.07%
Effective Date	Jun. 23, 2006
Expiration Date	Jul. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(203,000)		(435,000)
Swap 4 [Member]
Derivative [Line Items]
Notional Amount	8,492,000	[2]	8,480,000	[2]
Type of Derivative	Swap
Strike Rate	3.989%
Effective Date	Jul. 27, 2006
Expiration Date	Oct. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(255,000)		(432,000)
Swap 5 [Member]
Derivative [Line Items]
Notional Amount			39,483,000	[2]
Type of Derivative	Swap
Strike Rate	3.776%
Effective Date	Dec. 05, 2006
Expiration Date	Jan. 18, 2012	[3]
Fair Value at Significant Other Observable Inputs (Level 2)			(41,000)
Swap 6 [Member]
Derivative [Line Items]
Notional Amount			33,946,000	[2]
Type of Derivative	Swap
Strike Rate	4.00%
Effective Date	Dec. 20, 2006
Expiration Date	Jan. 18, 2012	[3]
Fair Value at Significant Other Observable Inputs (Level 2)			(38,000)
Swap 7 [Member]
Derivative [Line Items]
Notional Amount	39,579,000	[2]	38,883,000	[2]
Type of Derivative	Swap
Strike Rate	2.703%
Effective Date	Dec. 03, 2009
Expiration Date	Sep. 04, 2014
Fair Value at Significant Other Observable Inputs (Level 2)	(1,617,000)		(1,592,000)
Swap 8 [Member]
Derivative [Line Items]
Notional Amount	410,905,000	[4]
Type of Derivative	Swap
Strike Rate	0.717%
Effective Date Description	Various
Expiration Date Description	Various
Fair Value at Significant Other Observable Inputs (Level 2)	(3,642,000)
Swap 9 [Member]
Derivative [Line Items]
Notional Amount	155,099,000	[5]
Type of Derivative	Swap
Strike Rate	0.925%
Effective Date	Jul. 06, 2012
Expiration Date	Apr. 18, 2017	[6]
Fair Value at Significant Other Observable Inputs (Level 2)	(1,131,000)
Cap 1 [Member]
Derivative [Line Items]
Notional Amount			16,163,000
Type of Derivative	Cap
Strike Rate	4.00%
Effective Date	Jun. 24, 2009
Expiration Date	Jun. 25, 2012	[6]
Termination date	April 27, 2012
Date paid in full	Apr. 26, 2012
Cap 2 [Member]
Derivative [Line Items]
Notional Amount			$ 20,500,000
Type of Derivative	Cap
Strike Rate	4.00%
Effective Date	Aug. 04, 2010
Expiration Date	Jun. 15, 2013	[7]
Termination date	May 9, 2012
Date paid in full	May 04, 2012
GBP [Member]
Derivative [Line Items]
Exchange rate	1.63		1.55
EUR [Member]
Derivative [Line Items]
Exchange rate	1.32		1.30
SGD [Member]
Derivative [Line Items]
Exchange rate	0.82

[1] Translation to U.S. dollars is based on exchange rate of $1.63 to £1.00 as of December 31, 2012 and $1.55 to £1.00 as of December 31, 2011.

[2] Translation to U.S. dollars is based on exchange rate of $1.32 to €1.00 as of December 31, 2012 and $1.30 to €1.00 as of December 31, 2011.

[3] The swap agreements were terminated as the mortgage loans were paid in full at maturity in January 2012.

[4] Represents the U.S. Dollar tranche of the unsecured term loan.

[5] Represents the Singapore Dollar tranche of the unsecured term loan. Translation to U.S. dollars is based on exchange rate of $0.82 to 1.00 SGD as of December 31, 2012.

[6] This cap agreement was terminated on April 27, 2012 as the mortgage loan was paid in full on April 26, 2012.

[7] This cap agreement was terminated on May 9, 2012 as the loans were paid in full on May 4, 2012.