Indebtedness and Derivative Financial Instruments - Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Book value
Fair Value of Debt
Fair value of fixed-rate mortgages and unsecured indebtedness	$ 22,100	$ 20,400
Fixed-rate mortgages and unsecured indebtedness | Fair value
Fair Value of Debt
Fair value of fixed-rate mortgages and unsecured indebtedness	$ 22,703	$ 21,331
Mortgage
Fair Value of Debt
Weighted average discount rates assumed in calculation of fair value for debt (as a percent)	4.12%	3.46%
Unsecured Debt
Fair Value of Debt
Weighted average discount rates assumed in calculation of fair value for debt (as a percent)	3.83%	3.59%