Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Derivative [Line Items]
Lender accelerated indebtedness repayment threshold amount	$ 75,000,000
Fair value of effective cash flow hedges	(7,100,000)			(8,700,000)
Ineffective portion of cash flow hedges	0		0
Reclassified as increase to interest expense	5,200,000
Swap 1 [Member]
Derivative [Line Items]
Notional Amount	65,085,000	[1]		69,612,000	[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)	(1,068,000)			(1,552,000)
Swap 2 [Member]
Derivative [Line Items]
Notional Amount	12,904,000	[2]		13,335,000	[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)	(144,000)			(275,000)
Swap 3 [Member]
Derivative [Line Items]
Notional Amount	9,337,000	[2]		9,649,000	[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)	(107,000)			(203,000)
Swap 4 [Member]
Derivative [Line Items]
Notional Amount	8,217,000	[2]		8,492,000	[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)	(168,000)			(255,000)
Swap 5 [Member]
Derivative [Line Items]
Notional Amount	38,457,000	[2]		39,579,000	[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)	(1,295,000)			(1,617,000)
Swap 6 [Member]
Derivative [Line Items]
Notional Amount	410,905,000	[3]		410,905,000	[3]
Type of Derivative	Swap
Strike Rate	0.717%
Fair Value at Significant Other Observable Inputs (Level 2)	(3,293,000)			(3,642,000)
Swap 7 [Member]
Derivative [Line Items]
Notional Amount	152,786,000	[4]		155,099,000	[4]
Type of Derivative	Swap
Strike Rate	0.925%
Effective Date	Jul. 06, 2012
Expiration Date	Apr. 18, 2017
Fair Value at Significant Other Observable Inputs (Level 2)	(984,000)			(1,131,000)
Cap 1 [Member]
Derivative [Line Items]
Notional Amount	697,691,000			706,671,000
Fair Value at Significant Other Observable Inputs (Level 2)	$ (7,059,000)			$ (8,675,000)
GBP [Member]
Derivative [Line Items]
Exchange rate	1.52			1.63
EUR [Member]
Derivative [Line Items]
Exchange rate	1.28			1.32
SGD [Member]
Derivative [Line Items]
Exchange rate	0.81			0.82

[1] Translation to U.S. dollars is based on exchange rate of $1.52 to £1.00 as of March 31, 2013 and $1.63 to £1.00 as of December 31, 2012.

[2] Translation to U.S. dollars is based on exchange rate of $1.28 to €1.00 as of March 31, 2013 and $1.32 to €1.00 as of December 31, 2012.

[3] Represents the U.S. dollar tranche of the unsecured term loan.

[4] Represents the Singapore dollar tranche of the unsecured term loan. Translation to U.S. dollars is based on exchange rate of $0.81 to 1.00 SGD as of March 31, 2013 and $0.82 to 1.00 SGD as of December 31, 2012.