Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Derivative [Line Items]
Interest Expense	$ 49,146		$ 47,583	$ 96,520		$ 95,661
Notional Amount	712,918			712,918			560,945
Fair Value at Significant Other Observable Inputs (Level 2)	(4,327)			(4,327)			55
Forward Contracts [Member]
Derivative [Line Items]
Notional Amount	150,000	[1]		150,000	[1]
Type of Derivative				Forward-starting Swap
Strike Rate	2.091%			2.091%
Effective Date				Jul. 15, 2014
Expiration Date				Jul. 15, 2019
Fair Value at Significant Other Observable Inputs (Level 2)	(2,871)			(2,871)
Currently Paying Contracts [Member]
Derivative [Line Items]
Notional Amount	562,918			562,918			560,945
Fair Value at Significant Other Observable Inputs (Level 2)	(1,456)			(1,456)			55
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedge Adjustments [Member]
Derivative [Line Items]
Interest Expense				5,800
Swap 1 [Member] | Currently Paying Contracts [Member]
Derivative [Line Items]
Notional Amount	410,905	[2]		410,905	[2]		410,905	[2]
Type of Derivative				Swap
Strike Rate	0.717%			0.717%
Fair Value at Significant Other Observable Inputs (Level 2)	(1,080)			(1,080)			(76)
Swap 2 [Member] | Currently Paying Contracts [Member]
Derivative [Line Items]
Notional Amount	152,013	[3]		152,013	[3]		150,040	[3]
Type of Derivative				Swap
Strike Rate	0.925%			0.925%
Effective Date				Jul. 17, 2012
Expiration Date				Apr. 18, 2017
Fair Value at Significant Other Observable Inputs (Level 2)	$ (376)			$ (376)			$ 131
SGD [Member]
Derivative [Line Items]
Exchange rate	0.80			0.80			0.79

[1] In January 2014, we entered into a forward-starting swap agreement with a notional amount of USD $150.0 million for a future debt issuance with a tenor of five years or greater, which initially was expected to occur on July 15, 2014. The forward-starting swap has a mandatory early termination date of January 15, 2015 to cash settle the swap.

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

[3] Represents a portion of the Singapore dollar tranche of the unsecured term loan. Translation to U.S. dollars is based on exchange rate of $0.80 to 1.00 SGD as of June 30, 2014 and $0.79 to 1.00 SGD as of December 31, 2013.