Permanent and Temporary Equity (Details) (USD $)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2013	Dec. 31, 2013
Capital Stock [Abstract]
Equity Offering Program, Authorized Amount				$ 750,000,000
Number of shares of common stock issued and sold in an underwritten public offering		5,980,000		2,069,200
Aggregate Proceeds from Issuance of common stock		342,500,000	35,400,000	141,500,000
Sales Agent Commissions and Fees for Common Stock Sales				2,100,000
Equity Offering Program, Remaining Authorized Offering Amount			606,400,000	606,400,000
Stock repurchased, shares	3,700,000
Excess Share Provision
Percentage of beneficial ownership of common stock acquired used as threshold for determining excess shares			9.00%	9.00%
Percentage of beneficial ownership of preferred stock acquired used as threshold for determining excess shares			9.90%	9.90%
Maximum deferral payment period of purchase price for excess shares (in years)				5 years
Distribution Reinvestment and Stock Purchase Plan
Discount on purchase price of common stock to shareholders reinvesting their dividends and/or make optional cash purchases through the DRIP (as a percent)			1.00%	1.00%
Accumulated Other Comprehensive Income
Foreign currency translation	23,441,000		18,019,000	18,019,000
Unrealized gain (loss) on marketable debt securities	807,000		(216,000)	(216,000)
Other	(894,000)		1,856,000	1,856,000
Total accumulated other comprehensive income	$ 23,354,000		$ 19,659,000	$ 19,659,000