Unsecured Debt - Senior Unsecured Notes (Details) - USD ($)	Dec. 31, 2017	Apr. 30, 2017	Dec. 31, 2016	Apr. 30, 2014	Apr. 30, 2013	Sep. 30, 2012
Debt Instrument [Line Items]
Debt instrument, face amount	$ 2,900,000,000		$ 2,850,000,000
Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 0		300,000,000
Unsecured Bonds 5.500% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%			5.50%
Unsecured Bonds 5.200% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000		300,000,000
Unsecured Bonds 5.200% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.20%
Unsecured Bonds 3.625% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000		300,000,000
Unsecured Bonds 3.625% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.625%					3.625%
Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000		300,000,000
Unsecured Bonds 3.375% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%
Unsecured Bonds 3.25% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000		300,000,000
Unsecured Bonds 3.25% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.25%				3.25%
Unsecured Bonds 3.875% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 400,000,000		400,000,000
Unsecured Bonds 3.875% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount				$ 400,000,000
Interest rate, stated percentage	3.875%			3.875%
Unsecured Bonds 3.500% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 500,000,000		500,000,000
Unsecured Bonds 3.500% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.50%
Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 450,000,000		450,000,000
Unsecured Bonds 3.375% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%
Unsecured Bonds 3.625% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 350,000,000		$ 0
Unsecured Bonds 3.625% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 350,000,000
Interest rate, stated percentage	3.625%