Stockholders' Equity (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) vote shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Nov. 20, 2015 USD ($)	Mar. 31, 2015 USD ($) shares	Nov. 30, 2014 USD ($)	Aug. 31, 2011 USD ($)
Class of Stock [Line Items]
Dividends, Common Stock, Cash	$ 103	$ 97	$ 92
Common Stock, Voting Rights, Number of Votes Per Share | vote	1
Common stock, sinking fund	$ 0
2011 Program [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized amount							$ 100
Stock repurchased 2011 program, shares repurchased | shares					1,944,161
Aggregate purchase price of common stock repurchased					$ 100
2014 Program [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized amount				$ 150		$ 250
Stock repurchased 2014 program, shares repurchase | shares	0	1,511,109
Ordinary Dividend [Member]
Class of Stock [Line Items]
Dividends, Common Stock, Cash	$ 103	$ 97	$ 92