Unsecured Debt - Senior Unsecured Notes (Details) - USD ($)	Dec. 31, 2019	Oct. 31, 2019	Aug. 31, 2019	Mar. 31, 2019	Feb. 28, 2019	Dec. 31, 2018	Mar. 31, 2018	Apr. 30, 2017	Apr. 30, 2014	Apr. 30, 2013	Sep. 30, 2012
Debt Instrument [Line Items]
Debt instrument face amount	$ 4,250,000,000					$ 3,200,000,000
Unsecured Bonds 5.200%
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000					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	$ 300,000,000					300,000,000
Unsecured Bonds 3.625% | Senior Notes
Debt Instrument [Line Items]
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					0
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					0
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 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%