Capital Stock (Details 2) (Class A, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended	1 Months Ended					12 Months Ended
	Jul. 31, 2011	Dec. 31, 2010	Jul. 31, 2011 Long-term incentive plan	Jun. 30, 2010 Long-term incentive plan	Jul. 31, 2011 Other stock-based awards	Jul. 31, 2010 Other stock-based awards	Oct. 31, 2011 Stock repurchase program	Dec. 31, 2011 Stock repurchase program	Dec. 31, 2010 Stock repurchase program
Stock repurchases
Shares repurchased (in shares)	64,095	44,206	57,695	39,406	6,400	4,800		2,200,000	1,100,000
Total cost of shares repurchased	$ 4.8	$ 2.0	$ 4.3	$ 1.8	$ 0.5	$ 0.2		$ 155.0	$ 57.0
Amount of authorized stock repurchase approved for continuation							150
Approximate amount of repurchase authority remaining under stock repurchase program								$ 138
Repurchase price per share (in dollars per share)			$ 73.72	$ 45.92	$ 73.74	$ 48.75