Permanent and Temporary Equity (Details) (USD $)	1 Months Ended		12 Months Ended		1 Months Ended
	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2015
Capital Stock [Abstract]
Equity offering program, authorized amount			$ 750,000,000
Equity offering program, remaining authorized offering amount			360,400,000
Stock repurchased, shares	3,700,000
Sale of Stock [Line Items]
Number of shares of common stock issued and sold in an underwritten public offering		5,980,000	3,381,678	2,069,200
Aggregate proceeds from issuance of common stock		342,500,000	242,300,000	141,500,000
Sales agent commissions and fees for common stock sales			3,700,000	2,100,000
Excess Share Provision
Percentage of beneficial ownership of common stock acquired used as threshold for determining excess shares			9.00%
Percentage of beneficial ownership of preferred stock acquired used as threshold for determining excess shares			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%
Accumulated Other Comprehensive Income
Foreign currency translation			866,000	18,019,000
Unrealized gain (loss) on marketable securities			6,785,000	(216,000)
Other			5,470,000	1,856,000
Total accumulated other comprehensive income			13,121,000	19,659,000
Subsequent Event
Sale of Stock [Line Items]
Number of shares of common stock issued and sold in an underwritten public offering					3,750,202
Aggregate proceeds from issuance of common stock					285,800,000
Sales agent commissions and fees for common stock sales					$ 4,400,000