Financial Instruments - Treasury Locks (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013	Dec. 31, 2011	Dec. 31, 2009
Derivative [Line Items]
Notional Amounts	$ 11,734	$ 3,210
December 2008 Treasury Lock
Derivative [Line Items]
Debt instrument face amount	$ 600
Interest rate	4.50%
Notional Amounts	$ 500
Derivatives Designated as Hedging Instruments - Cash Flow | Treasury Rate Locks
Derivative [Line Items]
Original Gain / (Loss)			$ (2)	$ (11)
Unrecognized Gain / (Loss)	(3)	(2)
Less: income taxes	1	1
After- tax amounts	(2)	(1)
Proceeds from Hedge, Financing Activities					$ 16
Derivatives Designated as Hedging Instruments - Cash Flow | December 2008 Treasury Lock
Derivative [Line Items]
Unrecognized Gain / (Loss)	0	3
Derivatives Designated as Hedging Instruments - Cash Flow | Aug 2011Treasury Lock
Derivative [Line Items]
Unrecognized Gain / (Loss)	(3)	(4)
Derivatives Designated as Hedging Instruments - Cash Flow | Jul 2012 Treasury Lock
Derivative [Line Items]
Unrecognized Gain / (Loss)	$ 0	$ (1)