Disclosure about Fair Value of Financial Instruments (Details)	Mar. 31, 2014 USD ($)	Mar. 31, 2014 CAD	Mar. 31, 2014 GBP (£)	Dec. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Mar. 31, 2014 Mortgage Loans on Real Estate [Member] USD ($)	Dec. 31, 2013 Mortgage Loans on Real Estate [Member] USD ($)	Mar. 31, 2014 Other Real Estate Loans Receivable [Member] USD ($)	Dec. 31, 2013 Other Real Estate Loans Receivable [Member] USD ($)
Financial Assets:
Loans receivable, Carrying Amount	$ 351,401,000			$ 332,146,000			$ 154,320,000	$ 146,987,000	$ 197,081,000	$ 185,159,000
Loans receivable, Fair value							157,006,000	148,088,000	201,123,000	188,920,000
Available-for-sale equity investments, Carrying Amount	1,760,000			1,211,000
Available-for-sale equity investments, Fair Value	1,760,000			1,211,000
Cash and cash equivalents, Carrying Amount	185,928,000			158,780,000	269,842,000	1,033,764,000
Cash and cash equivalents, Fair Value	185,928,000			158,780,000
Foreign currency forward contract, asset	17,130,000			0
Interest rate derivative, Fair Value, Asset	25,000			38,000
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	562,000,000	0	0	130,000,000	710,000,000
Borrowings under unsecured lines of credit arrangements, Fair Value	562,000,000			130,000,000
Senior unsecured notes, Carrying Amount	7,377,789,000			7,379,308,000
Senior unsecured notes, Fair Value	7,987,921,000			7,743,730,000
Secured debt, Carrying Amount	2,917,314,000			3,058,248,000
Secured debt, Fair Value	3,030,057,000			3,168,775,000
Foreign currency forward contact, liability	$ 13,440,000			$ 11,637,000