Disclosure about Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial Assets:
Loans receivable, Carrying Amount	$ 380,169,000	$ 332,146,000
Available-for-sale equity investments, Carrying Amount	0	1,211,000
Available-for-sale equity investments, Fair Value	0	1,211,000
Cash and cash equivalents, Carrying Amount	473,726,000	158,780,000	1,033,764,000	163,482,000
Cash and cash equivalents	473,726,000	158,780,000
Foreign Currency Forward Contracts	57,087,000
Financial Liabilities:
Borrowings under unsecured lines of credit arrangements, Carrying Amount	0	130,000,000
Borrowings under unsecured lines of credit arrangements, Fair Value	0	130,000,000
Senior unsecured notes, Carrying Amount	7,766,251,000	7,379,308,000
Senior unsecured notes, Fair Value	8,613,702,000	7,743,730,000
Secured debt, Carrying Amount	2,977,713,000	3,058,248,000
Secured debt, Fair Value	3,053,067,000	3,168,775,000
Interest rate swap agreements, Carrying Amount	0	0
Interest rate swap agreements, Fair Value	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	1,495,000
Redeemable Operating Partnership Unit Holder Interests	46,722,000	0
Mortgage Loans [Member]
Financial Assets:
Loans receivable, Carrying Amount	188,651,000	146,987,000
Loans receivable, Fair value	194,935,000	148,088,000
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	191,518,000	185,159,000
Loans receivable, Fair value	$ 195,375,000	$ 188,920,000