Borrowing Arrangements (Summary of Senior Notes Payable and Other Debt) (Details) - USD ($) $ in Thousands	1 Months Ended				3 Months Ended	12 Months Ended
	Aug. 31, 2018	Jul. 31, 2018	Feb. 28, 2018	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2017
Debt Instrument [Line Items]
Derivative notional amount									$ 275,000
Derivative gain			$ 10,000		$ 800
Proceeds from issuance of debt						$ 2,549,473	$ 1,111,649	$ 893,218
Long-term debt, gross						10,829,702	11,365,633
Debt issuance costs, net						(69,615)	(73,093)
Unamortized fair value adjustment						(1,163)	12,139
Debt instrument, unamortized discount						(25,225)	(28,617)
Long-term Debt						10,733,699	11,276,062
Unsecured revolving credit facilities
Debt Instrument [Line Items]
Long-term debt, gross						765,919	535,832
Secured revolving construction credit facility due 2022
Debt Instrument [Line Items]
Long-term debt, gross						$ 90,488	2,868
2.00% Senior Notes, Due 2018
Debt Instrument [Line Items]
Interest rate			2.00%			2.00%
Long-term debt, gross						$ 0	700,000
4.00% Senior Notes, Due 2019
Debt Instrument [Line Items]
Interest rate						4.00%
Long-term debt, gross						$ 0	600,000
3.00% Senior Notes, Series A due 2019
Debt Instrument [Line Items]
Interest rate						3.00%
Long-term debt, gross						$ 293,319	318,041
2.70% Senior Notes, Due 2020
Debt Instrument [Line Items]
Interest rate						2.70%
Long-term debt, gross						$ 500,000	500,000
Unsecured term loan due 2020
Debt Instrument [Line Items]
Repayments of long-term debt		$ 900,000
Long-term debt, gross						$ 0	900,000
4.75% Senior Notes due 2021
Debt Instrument [Line Items]
Interest rate						4.75%
Long-term debt, gross						$ 0	700,000
4.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate						4.25%
Long-term debt, gross						$ 600,000	600,000
3.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate						3.25%
Long-term debt, gross						$ 500,000	500,000
3.30% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate						3.30%
Long-term debt, gross						$ 183,325	198,776
Unsecured Term Loan Due 2023
Debt Instrument [Line Items]
Long-term debt, gross		300,000				$ 300,000	0
3.125% Senior Notes due 2023
Debt Instrument [Line Items]
Interest rate						3.125%
Long-term debt, gross						$ 400,000	400,000
3.10% Senior Notes due 2023
Debt Instrument [Line Items]
Interest rate				3.10%	3.10%	3.10%
Public offering price as percent of par				99.28%	99.28%
Proceeds from issuance of debt				$ 397,100
Long-term debt, gross				$ 400,000	$ 400,000	$ 400,000	400,000
2.55% Senior Notes, Series D due 2023
Debt Instrument [Line Items]
Interest rate						2.55%
Long-term debt, gross						$ 201,657	218,653
Unsecured Term Loan Due 2024
Debt Instrument [Line Items]
Long-term debt, gross		$ 600,000				$ 600,000	0
3.75% Senior Notes due 2024
Debt Instrument [Line Items]
Interest rate						3.75%
Long-term debt, gross						$ 400,000	400,000
4.125% Senior Notes, Series B due 2024
Debt Instrument [Line Items]
Interest rate						4.125%
Long-term debt, gross						$ 183,324	198,776
3.50% Senior Notes due 2025
Debt Instrument [Line Items]
Interest rate						3.50%
Long-term debt, gross						$ 600,000	600,000
4.125% Senior Notes due 2026
Debt Instrument [Line Items]
Interest rate						4.125%
Long-term debt, gross						$ 500,000	500,000
3.25% Senior Notes due 2026
Debt Instrument [Line Items]
Interest rate						3.25%
Long-term debt, gross						$ 450,000	450,000
3.85% Senior Notes due 2027
Debt Instrument [Line Items]
Interest rate				3.85%	3.85%	3.85%
Long-term debt, gross						$ 400,000	400,000
4.00% Senior Notes due 2028
Debt Instrument [Line Items]
Interest rate			4.00%			4.00%
Public offering price as percent of par			99.23%
Proceeds from issuance of debt			$ 645,000
Long-term debt, gross			$ 650,000			$ 650,000	0
4.40% Senior Notes due 2029
Debt Instrument [Line Items]
Interest rate	4.40%					4.40%
Derivative gain	$ 4,400
Public offering price as percent of par	99.95%
Proceeds from issuance of debt	$ 749,700
Long-term debt, gross	$ 750,000					$ 750,000	0
6.90% Senior Notes due 2037
Debt Instrument [Line Items]
Interest rate						6.90%
Long-term debt, gross						$ 52,400	52,400
6.59% Senior Notes due 2038
Debt Instrument [Line Items]
Interest rate						6.59%
Long-term debt, gross						$ 22,823	22,973
5.45% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate						5.45%
Long-term debt, gross						$ 258,750	258,750
5.70% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate						5.70%
Long-term debt, gross						$ 300,000	300,000
4.375% Senior Notes due 2045
Debt Instrument [Line Items]
Interest rate						4.375%
Long-term debt, gross						$ 300,000	300,000
Mortgage loans and other
Debt Instrument [Line Items]
Long-term debt, gross						1,127,697	1,308,564
Borrowings originally denominated in CAD | Unsecured revolving credit facilities
Debt Instrument [Line Items]
Long-term debt, gross						23,100	28,700
Borrowings originally denominated in GBP | Unsecured revolving credit facilities
Debt Instrument [Line Items]
Long-term debt, gross						$ 27,800	$ 31,100