Unsecured Debt - Senior Unsecured Notes (Details) - USD ($)	Dec. 31, 2020	Aug. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Oct. 31, 2019	Aug. 31, 2019	Mar. 31, 2019	Feb. 28, 2019	Mar. 31, 2018	Apr. 30, 2017	Apr. 30, 2014	Apr. 30, 2013
Debt Instrument [Line Items]
Debt instrument face amount	$ 4,900,000,000			$ 4,250,000,000
Unsecured Bonds 5.200%
Debt Instrument [Line Items]
Debt instrument face amount	$ 0			300,000,000
Unsecured Bonds 5.200% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	5.20%
Unsecured Bonds 3.625%
Debt Instrument [Line Items]
Debt instrument face amount	$ 0			300,000,000
Unsecured Bonds 3.625% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount		$ 300,000,000.0
Coupon Rate	3.625%	3.625%
Unsecured Bonds 3.375%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000
Unsecured Bonds 3.375% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	3.375%
Unsecured Bonds 3.250%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000
Unsecured Bonds 3.250% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	3.25%											3.25%
Unsecured Bonds 3.875%
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000			400,000,000
Unsecured Bonds 3.875% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount											$ 400,000,000.0
Coupon Rate	3.875%										3.875%
Unsecured Bonds 3.500%
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000			500,000,000
Unsecured Bonds 3.500% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	3.50%
Unsecured Bonds 3.375%
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000.0			450,000,000
Unsecured Bonds 3.375% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	3.375%
Unsecured Bonds 3.625%
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000			350,000,000
Unsecured Bonds 3.625% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount										$ 350,000,000.0
Coupon Rate	3.625%
Unsecured Bonds 4.000%
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000			500,000,000
Unsecured Bonds 4.000% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount							$ 150,000,000.0	$ 350,000,000.0
Coupon Rate	4.00%							4.00%
Unsecured Bonds 3.000%
Debt Instrument [Line Items]
Debt instrument face amount	$ 550,000,000			550,000,000
Unsecured Bonds 3.000% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount					$ 150,000,000.0	$ 400,000,000.0
Coupon Rate	3.00%					3.00%
Unsecured Bonds 1.650%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			0
Unsecured Bonds 1.650% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	1.65%
Unsecured Bonds 2.650%
Debt Instrument [Line Items]
Debt instrument face amount	$ 650,000,000			0
Unsecured Bonds 2.650% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount			$ 150,000,000.0
Coupon Rate	2.65%
Unsecured Bonds 4.500%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000
Unsecured Bonds 4.500% | Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount									$ 300,000,000.0
Coupon Rate	4.50%								4.50%
Unsecured Bonds 2.650%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			$ 0
Unsecured Bonds 2.650% | Senior Notes
Debt Instrument [Line Items]
Coupon Rate	2.65%