Financial Instruments - Schedule of Treasury Rate Lock Contracts (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2011
Derivative [Line Items]
Basic Earnings Per Share (in dollars per share)	$ 0.95	$ 1.61
$500 million 3.00% fixed-rate notes that mature in 2021
Derivative [Line Items]
Face amount	$ 500
Interest rate	3.00%
Cash Flow Hedging | Interest rate lock commitments
Derivative [Line Items]
Original Gain / (Loss)				$ 11
Cash Flow Hedging | $500 million 3.00% fixed-rate notes that mature in 2021
Derivative [Line Items]
Unrecognized Gain / (Loss)	$ (3)		$ (3)
Cash Flow Hedging | Treasury Lock
Derivative [Line Items]
Unrecognized Gain / (Loss)	(3)		(3)
Less: income taxes	1		1
After- tax amounts	$ (2)		$ (2)