Notes Payable - General (Details) - USD ($) $ in Thousands	1 Months Ended
	Apr. 30, 2018	Dec. 31, 2017	Mar. 31, 2017	Sep. 30, 2014	Oct. 31, 2012	Jun. 30, 2011	Mar. 31, 2005	Jun. 30, 2018	Jan. 31, 2018
Debt
Notes payable, net		$ 5,230,244						$ 5,374,963
Notes Payable
Debt
Notes payable, gross		5,250,000						5,400,000
Unamortized net original issuance premiums and deferred financing costs		(20,000)						(25,000)
Notes payable, net		$ 5,230,000						5,375,000
Issuance of Debt	$ 500,000
2.000% notes, issued in October 2012 and due in January 2018 | Notes Payable
Debt
Stated interest rate (as a percent)		2.00%							2.00%
Notes payable, gross		$ 350,000						$ 0
5.750% notes, issued in June 2010 and due in January 2021 | Notes Payable
Debt
Stated 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
Stated 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
Stated 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
Stated interest rate (as a percent)		3.875%						3.875%
Notes payable, gross		$ 350,000						$ 350,000
3.875% notes, issued in April 2018 and due in April 2025 | Notes Payable
Debt
Stated interest rate (as a percent)		3.875%						3.875%
Notes payable, gross		$ 0						$ 500,000
4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026 | Notes Payable
Debt
Stated 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
Stated interest rate (as a percent)			4.125%
Issuance of Debt			$ 400,000
3.000% notes, issued in October 2016 and due in January 2027 | Notes Payable
Debt
Stated 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
Stated 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
Stated 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
Stated 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