Notes Payable - General (Details) - USD ($) $ in Thousands	1 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Sep. 30, 2014	Oct. 31, 2012	Jun. 30, 2011	Mar. 31, 2005	Mar. 31, 2018
Debt
Notes payable, net	$ 5,230,244						$ 4,879,628
Notes Payable
Debt
Notes payable, gross	5,250,000						4,900,000
Unamortized net original issuance premiums and deferred financing costs	(20,000)						(20,000)
Notes payable, net	$ 5,230,000						$ 4,880,000
2.000% notes, issued in October 2012 and due in January 2018 | Notes Payable
Debt
Interest rate (as a percent)	2.00%
Notes payable, gross	$ 350,000
5.750% notes, issued in June 2010 and due in January 2021 | Notes Payable
Debt
Interest rate (as a percent)	5.75%						5.75%
Notes payable, gross	$ 250,000						$ 250,000
3.250% notes, $450 issued in October 2012 and $500 issued in December 2017, both due in October 2022 | Notes Payable
Debt
Interest rate (as a percent)	3.25%						3.25%
Notes payable, gross	$ 950,000						$ 950,000
3.250% notes, issued in October 2012 and due in October 2022 | Notes Payable
Debt
Issuance of Debt				$ 450,000
3.250% notes, issued in December 2017 and due in October 2022 | Notes Payable
Debt
Issuance of Debt	$ 500,000
4.650% notes, issued in July 2013 and due in August 2023 | Notes Payable
Debt
Interest rate (as a percent)	4.65%						4.65%
Notes payable, gross	$ 750,000						$ 750,000
3.875% notes, issued in June 2014 and due in July 2024 | Notes Payable
Debt
Interest rate (as a percent)	3.875%						3.875%
Notes payable, gross	$ 350,000						$ 350,000
4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026 | Notes Payable
Debt
Interest rate (as a percent)	4.125%						4.125%
Notes payable, gross	$ 650,000						$ 650,000
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
Interest rate (as a percent)	3.00%						3.00%
Notes payable, gross	$ 600,000						$ 600,000
3.650% notes, issued in December 2017 and due in January 2028 | Notes Payable
Debt
Interest rate (as a percent)	3.65%						3.65%
Notes payable, gross	$ 550,000						$ 550,000
5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035 | Notes Payable
Debt
Interest rate (as a percent)	5.875%						5.875%
Notes payable, gross	$ 250,000						$ 250,000
5.875% bonds, issued in March 2005 and due in March 2035 | Notes Payable
Debt
Issuance of Debt						$ 100,000
5.875% bonds, issued in June 2011 and due in March 2035 | Notes Payable
Debt
Issuance of Debt					$ 150,000
4.650% notes, $300 issued in March 2017 and $250 issued in December 2017, both due in March 2047 | Notes Payable
Debt
Interest rate (as a percent)	4.65%						4.65%
Notes payable, gross	$ 550,000						$ 550,000
4.650% notes, issued in March 2017 and due in March 2047 | Notes Payable
Debt
Issuance of Debt		$ 300,000
4.650% notes, issued in December 2017 and due in March 2047 | Notes Payable
Debt
Issuance of Debt	$ 250,000