Unsecured Debt - Senior Unsecured Notes (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2014	Apr. 30, 2013	Sep. 30, 2012
Debt Instrument [Line Items]
Debt instrument, face amount	$ 2,850,000,000	$ 2,400,000,000
Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000	300,000,000
Unsecured Bonds 5.500% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	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			$ 400,000,000.0
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	$ 0
Unsecured Bonds 3.375% [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%