Notes Payable (Details) - USD ($) $ in Thousands	1 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2014	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt
Notes payable, net	$ 4,468,665			$ 4,468,665		$ 3,934,433
Principal payments on notes payable				$ 175,000	$ 275,000
Notes and bonds payable
Debt
Weighted average interest rate	4.30%			4.30%
Weighted average remaining years until maturity				7 years 10 months 24 days
Maturity of notes and bonds payable
2018	$ 350,000			$ 350,000
2019	550,000			550,000
2021	250,000			250,000
Thereafter	3,350,000			3,350,000
Totals	4,500,000			4,500,000
Notes Payable
Debt
Total principal amount	4,500,000			4,500,000		3,975,000
Unamortized original issuance discounts and deferred financing costs	(31,000)			(31,000)		(41,000)
Notes payable, net	$ 4,469,000			$ 4,469,000		3,934,000
5.375% notes, issued in September 2005 and due in September 2017 | Notes Payable
Debt
Total principal amount						$ 175,000
Interest rate (as a percent)	5.375%			5.375%		5.375%
Principal payments on notes payable	$ 175,000
2.000% notes, issued in October 2012 and due in January 2018 | Notes Payable
Debt
Total principal amount	$ 350,000			$ 350,000		$ 350,000
Interest rate (as a percent)	2.00%			2.00%		2.00%
6.750% notes, issued in September 2007 and due in August 2019 | Notes Payable
Debt
Total principal amount	$ 550,000			$ 550,000		$ 550,000
Interest rate (as a percent)	6.75%			6.75%		6.75%
5.750% notes, issued in June 2010 and due in January 2021 | Notes Payable
Debt
Total principal amount	$ 250,000			$ 250,000		$ 250,000
Interest rate (as a percent)	5.75%			5.75%		5.75%
3.250% notes, issued in October 2012 and due in October 2022 | Notes Payable
Debt
Total principal amount	$ 450,000			$ 450,000		$ 450,000
Interest rate (as a percent)	3.25%			3.25%		3.25%
4.650% notes, issued in July 2013 and due in August 2023 | Notes Payable
Debt
Total principal amount	$ 750,000			$ 750,000		$ 750,000
Interest rate (as a percent)	4.65%			4.65%		4.65%
3.875% notes, issued in June 2014 and due in July 2024 | Notes Payable
Debt
Total principal amount	$ 350,000			$ 350,000		$ 350,000
Interest rate (as a percent)	3.875%			3.875%		3.875%
4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026 | Notes Payable
Debt
Total principal amount	$ 650,000			$ 650,000		$ 250,000
Interest rate (as a percent)	4.125%			4.125%		4.125%
4.125% notes, issued in September 2014 and due in October 2026 | Notes Payable
Debt
Issuance of Debt			$ 250,000
4.125% notes, issued in March 2017 and due in October 2026 | Notes Payable
Debt
Issuance of Debt				$ 400,000
3.000% notes, issued in October 2016 and due in January 2027 | Notes Payable
Debt
Total principal amount	$ 600,000			$ 600,000		$ 600,000
Interest rate (as a percent)	3.00%			3.00%		3.00%
5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035 | Notes Payable
Debt
Total principal amount	$ 250,000			$ 250,000		$ 250,000
Interest rate (as a percent)	5.875%			5.875%		5.875%
5.875% bonds, issued in March 2005 and due in March 2035 | Notes Payable
Debt
Issuance of Debt				$ 100,000		$ 100,000
5.875% bonds, issued in June 2011 and due in March 2035 | Notes Payable
Debt
Issuance of Debt				150,000		$ 150,000
4.650% senior unsecured notes due 2047 | Notes Payable
Debt
Total principal amount	$ 300,000	$ 300,000		$ 300,000
Interest rate (as a percent)	4.65%	4.65%		4.65%
Percentage price paid to the investor		99.97%
Effective yield (as a percent)		4.65%
4.125% senior unsecured notes due 2026 | Notes Payable
Debt
Total principal amount		$ 400,000	$ 250,000
Interest rate (as a percent)		4.125%
Percentage price paid to the investor		102.98%
Effective yield (as a percent)		3.75%
Proceeds from issuance of debt		$ 705,200