Notes Payable - General (Details) - USD ($)	1 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Sep. 30, 2014	Oct. 31, 2012	Jun. 30, 2011	Mar. 31, 2005	Mar. 31, 2019	Dec. 31, 2018
Debt
Total principal amount							$ 5,377,712,000	$ 5,376,797,000
Notes and bonds payable
Debt
Total principal amount							5,400,000,000	5,400,000,000
Notes and bonds payable | 5.750% notes, issued in June 2010 and due in January 2021
Debt
Total principal amount							$ 250,000,000	$ 250,000,000
Interest rate (as a percent)							5.75%	5.75%
Notes and bonds payable | 3.250% notes, $450 issued in October 2012 and $500 issued in December 2017, both due in October 2022
Debt
Total principal amount							$ 950,000,000	$ 950,000,000
Interest rate (as a percent)							3.25%	3.25%
Notes and bonds payable | 3.250% notes, issued in October 2012 and due in October 2022
Debt
Issuance of debt				$ 450,000,000
Notes and bonds payable | 3.250% notes, issued in December 2017 and due in October 2022
Debt
Issuance of debt	$ 500,000,000
Notes and bonds payable | 4.650% notes, issued in July 2013 and due in August 2023
Debt
Total principal amount							$ 750,000,000	$ 750,000,000
Interest rate (as a percent)							4.65%	4.65%
Notes and bonds payable | 3.875% notes, issued in June 2014 and due in July 2024
Debt
Total principal amount							$ 350,000,000	$ 350,000,000
Interest rate (as a percent)							3.875%	3.875%
Notes and bonds payable | 3.875% notes, issued in April 2018 and due in April 2025
Debt
Total principal amount							$ 500,000,000	$ 500,000,000
Interest rate (as a percent)							3.875%	3.875%
Notes and bonds payable | 4.125% notes, $250 issued in September 2014 and $400 issued in March 2017, both due in October 2026
Debt
Total principal amount							$ 650,000,000	$ 650,000,000
Interest rate (as a percent)							4.125%	4.125%
Notes and bonds payable | 4.125% notes, issued in September 2014 and due in October 2026
Debt
Issuance of debt			$ 250,000,000
Notes and bonds payable | 4.125% notes, issued in March 2017 and due in October 2026
Debt
Issuance of debt		$ 400,000,000
Notes and bonds payable | 3.000% notes, issued in October 2016 and due in January 2027
Debt
Total principal amount							$ 600,000,000	$ 600,000,000
Interest rate (as a percent)							3.00%	3.00%
Notes and bonds payable | 3.650% notes, issued in December 2017 and due in January 2028
Debt
Total principal amount							$ 550,000,000	$ 550,000,000
Interest rate (as a percent)							3.65%	3.65%
Notes and bonds payable | 5.875% bonds, $100 issued in March 2005 and $150 issued in June 2011, both due in March 2035
Debt
Total principal amount							$ 250,000,000	$ 250,000,000
Interest rate (as a percent)							5.875%	5.875%
Notes and bonds payable | 5.875% bonds, issued in March 2005 and due in March 2035
Debt
Issuance of debt						$ 100,000,000
Notes and bonds payable | 5.875% bonds, issued in June 2011 and due in March 2035
Debt
Issuance of debt					$ 150,000,000
Notes and bonds payable | 4.650% notes, $300 issued in March 2017 and $250 issued in December 2017, both due in March 2047
Debt
Total principal amount							$ 550,000,000	$ 550,000,000
Interest rate (as a percent)							4.65%	4.65%
Notes and bonds payable | 4.650% notes, issued in March 2017 and due in March 2047
Debt
Issuance of debt		$ 300,000,000
Notes and bonds payable | 4.650% notes, issued in December 2017 and due in March 2047
Debt
Issuance of debt	$ 250,000,000