Notes Payable (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2014
Debt instrument
Total principal amount	$ 4,675,000	$ 4,675,000	$ 3,975,000
Unamortized original issuance discounts and deferred financing costs	(34,000)	(34,000)	(41,000)
Notes payable, net	4,641,094	4,641,094	3,934,433
Maturity of notes and bonds payable
2017	175,000	175,000
2018	350,000	350,000
2019	550,000	550,000
2021	250,000	250,000
Thereafter	3,350,000	3,350,000
Totals	4,675,000	4,675,000
5.375% notes, issued in September 2005 and due in September 2017
Debt instrument
Total principal amount	$ 175,000	$ 175,000	$ 175,000
Interest rate (as a percent)	5.375%	5.375%	5.375%
2.000% notes, issued in October 2012 and due in January 2018
Debt instrument
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
Debt instrument
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
Debt instrument
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
Debt instrument
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
Debt instrument
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
Debt instrument
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, all of which are due in October 2026
Debt instrument
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
Debt instrument
Issuance of Debt		$ 250,000	$ 250,000
4.125% notes, issued in March 2017 and due in October 2026
Debt instrument
Issuance of Debt		400,000
3.000% notes, issued in October 2016 and due in January 2027
Debt instrument
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, all of which are due in March 2035
Debt instrument
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
Debt instrument
Issuance of Debt		$ 100,000	$ 100,000
5.875% bonds, issued in June 2011 and due in March 2035
Debt instrument
Issuance of Debt		$ 150,000	$ 150,000
Notes and bonds payable
Debt instrument
Weighted average interest rate	4.40%	4.40%
Weighted average remaining years until maturity		8 years 1 month 6 days
4.650% senior unsecured notes due 2047
Debt instrument
Total principal amount	$ 300,000	$ 300,000
Interest rate (as a percent)	4.65%	4.65%	4.65%
Percentage price paid to the investor	99.97%	99.97%
Effective yield (as a percent)	4.65%	4.65%
4.125% senior unsecured notes due 2026
Debt instrument
Total principal amount	$ 400,000	$ 400,000		$ 250,000
Interest rate (as a percent)	4.125%	4.125%
Percentage price paid to the investor	102.98%	102.98%
Effective yield (as a percent)	3.75%	3.75%
Proceeds from issuance of debt	$ 705,200