Capital Stock (Narrative) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2016	Jun. 30, 2015	Feb. 28, 2014	Dec. 31, 2007
Equity, Class of Treasury Stock [Line Items]
Shares authorized	800,000,000
Common stock stated value per share (in dollars per share)	$ 0.01
Common shares outstanding	168,100,000	175,900,000
Treasury stock, number of shares retired		165,000,000
Stock repurchased	8,700,000	9,700,000	27,800,000
Preferred stock shares authorized	2,000,000
Preferred stock stated value per share (in dollars per share)	$ 1.00
Preferred stock, shares outstanding	0
1997 Authorization
Equity, Class of Treasury Stock [Line Items]
Stock repurchased	0	0	900,000
Stock repurchase program, covenant percentage	1.00%
2007 Authorization
Equity, Class of Treasury Stock [Line Items]
Stock repurchased	0	0	900,000
2014 Authorization
Equity, Class of Treasury Stock [Line Items]
Stock repurchased	0	0	20,000,000
2015 Authorization
Equity, Class of Treasury Stock [Line Items]
Stock repurchased	0	9,000,000	6,000,000
2016 Authorization
Equity, Class of Treasury Stock [Line Items]
Stock repurchased	8,700,000	700,000	0
Common Stock
Equity, Class of Treasury Stock [Line Items]
Treasury stock, number of shares retired		165,000,000
Stock repurchased	8,800,000
Weighted-average cost per share repurchased (in dollars per share)	$ 53
Treasury stock, shares, acquired from employees	200,000
Common Stock | 2007 Authorization
Equity, Class of Treasury Stock [Line Items]
Common stock repurchase authorization value							20,000,000
Common Stock | 2014 Authorization
Equity, Class of Treasury Stock [Line Items]
Common stock repurchase authorization value						20,000,000
Common Stock | 2015 Authorization
Equity, Class of Treasury Stock [Line Items]
Common stock repurchase authorization value					15,000,000
Common Stock | 2016 Authorization
Equity, Class of Treasury Stock [Line Items]
Common stock repurchase authorization value				20,000,000