Borrowing Arrangements (Summary of Senior Notes Payable and Other Debt) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt instruments
Principal amount outstanding	$ 10,872,371,000	$ 9,320,477,000
Unamortized fair value adjustment	41,853,000	69,611,000
Unamortized discounts	(26,132,000)	(25,096,000)
Senior notes payable and other debt	10,888,092,000	9,364,992,000
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding	919,099,000	376,343,000
Unsecured revolving credit facilities | Canadian dollars
Debt instruments
Principal amount outstanding	164,100,000	7,300,000
3.125% Senior Notes due 2015
Debt instruments
Principal amount outstanding	400,000,000	400,000,000
Interest rate (as a percent)	3.125%	3.125%
6% Senior Notes due 2015
Debt instruments
Principal amount outstanding	234,420,000	234,420,000
Interest rate (as a percent)	6.00%	6.00%
1.55% Senior Notes due 2016
Debt instruments
Principal amount outstanding	550,000,000	550,000,000
Interest rate (as a percent)	1.55%	1.55%
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding	300,000,000
Interest rate (as a percent)	1.25%
2.00% Senior Notes due 2018
Debt instruments
Principal amount outstanding	700,000,000	700,000,000
Interest rate (as a percent)	2.00%	2.00%
Unsecured term loan
Debt instruments
Amount outstanding	1,000,000,000
Unsecured term loan | Canadian dollars
Debt instruments
Principal amount outstanding	107,000,000
Unsecured term loan due 2018
Debt instruments
Principal amount outstanding	200,000,000	200,000,000
Unsecured term loan due 2019
Debt instruments
Principal amount outstanding	790,634,000	800,702,000
4.00% Senior Notes due 2019
Debt instruments
Principal amount outstanding	600,000,000	600,000,000
Interest rate (as a percent)	4.00%	4.00%
3.00% Senior Notes, Series A due 2019
Debt instruments
Principal amount outstanding	344,204,000
Interest rate (as a percent)	3.00%
2.700% Senior Notes due 2020
Debt instruments
Principal amount outstanding	500,000,000	500,000,000
Interest rate (as a percent)	2.70%	2.70%
4.750% Senior Notes due 2021
Debt instruments
Principal amount outstanding	700,000,000	700,000,000
Interest rate (as a percent)	4.75%	4.75%
4.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	600,000,000	600,000,000
Interest rate (as a percent)	4.25%	4.25%
3.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	500,000,000	500,000,000
Interest rate (as a percent)	3.25%	3.25%
3.750% Senior Notes due 2024
Debt instruments
Principal amount outstanding	400,000,000
Interest rate (as a percent)	3.75%
4.125% Senior Notes, Series B due 2024
Debt instruments
Principal amount outstanding	215,128,000
Interest rate (as a percent)	4.125%
6.90% Senior Notes due 2037
Debt instruments
Principal amount outstanding	52,400,000	52,400,000
Interest rate (as a percent)	6.90%	6.90%
6.59% Senior Notes due 2038
Debt instruments
Principal amount outstanding	22,973,000	22,973,000
Interest rate (as a percent)	6.59%	6.59%
5.45% Senior Notes due 2043
Debt instruments
Principal amount outstanding	258,750,000	258,750,000
Interest rate (as a percent)	5.45%	5.45%
5.70% Senior Notes due 2043
Debt instruments
Principal amount outstanding	300,000,000	300,000,000
Interest rate (as a percent)	5.70%	5.70%
Mortgage loans
Debt instruments
Principal amount outstanding	2,284,763,000	2,524,889,000
Repayments of secured debt	398,000,000	493,700,000	344,200,000
Mortgage loans held for sale
Debt instruments
Principal amount outstanding	$ 43,500,000	$ 13,100,000