Capital Stock - Schedule of Common Stock Outstanding (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common stock:
Balance, beginning of year	109.3	108.5	107.8
Activity under stock plans	0.4	0.8	0.7
Balance, end of year	109.7	109.3	108.5
Treasury stock:
Balance, beginning of year	25.7	23.7	18.2
Repurchased	0.4	2.0	5.5
Balance, end of year	26.1	25.7	23.7
Common stock outstanding	83.6	83.6	84.8