Stockholders' Equity Stockholders' Equity - Additional Information (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Stockholders' Equity Attributable to Parent [Abstract]
Stock Repurchase Program, Authorized Amount	$ 500	$ 500
Treasury Stock, Shares, Acquired	2.7	2.7
Repurchase of common stock	$ 31	$ 31	$ 0