Shareholders' Equity - Common Stock Rollforward (Details) - shares						6 Months Ended	12 Months Ended
	Jun. 19, 2019	Jun. 04, 2019	Apr. 09, 2019	Apr. 08, 2019	Jan. 03, 2019	Jun. 30, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity
Common stock, shares issued, beginning balance (in shares)						272,431,745
Common shares, beginning balance (in shares)						150,978,184
Common stock acquired - share repurchase (in shares)	2,963,512	879,199	3,593,453	290,765	5,059,449	(12,800,000)	(22,800,000)
Share-based compensation programs (in shares)						2,100,000	1,800,000
Common stock, shares issued, ending balance (in shares)						275,273,194	272,431,745
Common shares, ending balance (in shares)						140,329,307	150,978,184
Common Stock, Issued
Increase (Decrease) in Stockholders' Equity
Common stock, shares issued, beginning balance (in shares)						272,400,000	270,000,000.0
Common shares, beginning balance (in shares)						151,000,000.0	172,000,000.0
Common shares issued (in shares)						0	0
Common stock acquired - share repurchase (in shares)						0	0
Share-based compensation programs (in shares)						2,800,000	2,400,000
Common stock, shares issued, ending balance (in shares)						275,200,000	272,400,000
Common shares, ending balance (in shares)						140,300,000	151,000,000.0
Common Stock, Held in Treasury
Increase (Decrease) in Stockholders' Equity
Common shares, held in treasury, beginning balance (in shares)						(121,400,000)	(98,000,000.0)
Common shares issued (in shares)						0	0
Common stock acquired - share repurchase (in shares)						(12,800,000)	(22,800,000)
Share-based compensation programs (in shares)						(700,000)	(600,000)
Common shares, held in treasury, ending balance (in shares)						(134,900,000)	(121,400,000)