Capital Stock (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders Equity Note [Abstract]

Schedule of Common Stock Outstanding

Common stock outstanding as of December 31, 2017, 2016 and 2015 was as follows:

(Number of shares in millions)	2017		2016		2015
Common stock:
Balance, beginning of year	106.7			106.0	104.8
Activity under stock plans	1.1		0.7		1.2
Balance, end of year	107.8		106.7			106.0
Treasury stock:
Balance, beginning of year	15.3		12.5		9.3
Repurchased		2.9	2.8		3.2
Balance, end of year	18.2		15.3		12.5
Common stock outstanding	89.6		91.4		93.5