Derivative Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011		Jun. 30, 2011		Jun. 30, 2010	Dec. 31, 2010
Fair value of effective cash flow hedges	$ (5,700,000)		$ (5,700,000)			$ (8,000,000)
Ineffective portion of cash flow hedges	0		0		0
Reclassified as increase to interest expense			3,800,000
Notional Amount	267,612,000		267,612,000			274,042,000
Fair Value at Significant Other Observable Inputs (Level 2)	(5,692,000)		(5,692,000)			(7,997,000)
Swap 1 [Member]
Notional Amount						19,515,000	[1]
Type of Derivative			Swap
Strike Rate	4.944		4.944
Effective Date			Jul. 10, 2006
Expiration Date			Apr. 10, 2011
Fair Value at Significant Other Observable Inputs (Level 2)						(231,000)
Swap 2 [Member]
Notional Amount	68,747,000	[1]	68,747,000	[1]		66,858,000	[1]
Type of Derivative			Swap
Strike Rate	2.980		2.980
Effective Date			Apr. 06, 2009
Expiration Date			Nov. 30, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(2,606,000)		(2,606,000)			(2,471,000)
Swap 3 [Member]
Notional Amount	15,024,000	[2]	15,024,000	[2]		13,978,000	[2]
Type of Derivative			Swap
Strike Rate	3.981		3.981
Effective Date			May 17, 2006
Expiration Date			Jul. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(593,000)		(593,000)			(828,000)
Swap 4 [Member]
Notional Amount	10,870,000	[2]	10,870,000	[2]		10,113,000	[2]
Type of Derivative			Swap
Strike Rate	4.070		4.070
Effective Date			Jun. 23, 2006
Expiration Date			Jul. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(448,000)		(448,000)			(621,000)
Swap 5 [Member]
Notional Amount	9,566,000	[2]	9,566,000	[2]		8,900,000	[2]
Type of Derivative			Swap
Strike Rate	3.989		3.989
Effective Date			Jul. 27, 2006
Expiration Date			Oct. 18, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	(412,000)		(412,000)			(557,000)
Swap 6 [Member]
Notional Amount	44,534,000	[2]	44,534,000	[2]		41,430,000	[2]
Type of Derivative			Swap
Strike Rate	3.776		3.776
Effective Date			Dec. 05, 2006
Expiration Date			Jan. 18, 2012
Fair Value at Significant Other Observable Inputs (Level 2)	(516,000)		(516,000)			(1,129,000)
Swap 7 [Member]
Notional Amount	38,289,000	[2]	38,289,000	[2]		35,620,000	[2]
Type of Derivative			Swap
Strike Rate	4.000		4.000
Effective Date			Dec. 20, 2006
Expiration Date			Jan. 18, 2012
Fair Value at Significant Other Observable Inputs (Level 2)	(491,000)		(491,000)			(1,054,000)
Swap 8 [Member]
Notional Amount	43,506,000	[2]	43,506,000	[2]		40,152,000	[2]
Type of Derivative			Swap
Strike Rate	2.703		2.703
Effective Date			Dec. 03, 2009
Expiration Date			Sep. 04, 2014
Fair Value at Significant Other Observable Inputs (Level 2)	(630,000)		(630,000)			(1,139,000)
Cap 1 [Member]
Notional Amount	16,576,000		16,576,000			16,976,000
Type of Derivative			Cap
Strike Rate	4.000		4.000
Effective Date			Jun. 24, 2009
Expiration Date			Jun. 25, 2012
Fair Value at Significant Other Observable Inputs (Level 2)						3,000
Cap 2 [Member]
Notional Amount	20,500,000		20,500,000			20,500,000
Type of Derivative			Cap
Strike Rate	4.000		4.000
Effective Date			Aug. 04, 2010
Expiration Date			Jun. 15, 2013
Fair Value at Significant Other Observable Inputs (Level 2)	$ 4,000		$ 4,000			$ 30,000
Euro [Member]
Exchange rate	1.45		1.45			1.34
British Sterling [Member]
Exchange rate	1.61		1.61			1.56

[1] Translation to U.S. dollars is based on exchange rate of $1.61 to £1.00 as of June 30, 2011 and $1.56 to £1.00 as of December 31, 2010.

[2] Translation to U.S. dollars is based on exchange rate of $1.45 to €1.00 as of June 30, 2011 and $1.34 to €1.00 as of December 31, 2010.