Capital Stock - Schedule of Common Stock Outstanding (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 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