Indebtedness and Derivative Financial Instruments - Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Book value
Fair Value of Debt
Fixed-rate mortgages and unsecured indebtedness	$ 20,400	$ 19,400
Fixed-rate mortagages and unsecured indebtedness | Fair value
Fair Value of Debt
Fixed-rate mortgages and unsecured indebtedness	$ 21,331	$ 20,967
Mortgages
Fair Value of Debt
Weighted average discount rates assumed in calculation of fair value for debt (as a percent)	3.46%	3.02%
Unsecured Debt
Fair Value of Debt
Weighted average discount rates assumed in calculation of fair value for debt (as a percent)	3.59%	3.51%