Capital Stock (Details 2) - Class A - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Oct. 31, 2011
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 2.6	$ 7.3
Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)	22,247	60,332
Equity compensation plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 0.7	$ 0.4
Average price per share for shares purchased (in dollars per share)	$ 117.39	$ 111.55
Equity compensation plan | Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)	5,830	3,785
Stock compensation plan for outside directors
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 0.5	$ 1.2
Average price per share for shares purchased (in dollars per share)	$ 115.69	$ 121.85
Stock compensation plan for outside directors | Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)	4,576	9,663
Incentive compensation deferral plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 1.4
Average price per share for shares purchased (in dollars per share)	$ 119.14
Incentive compensation deferral plan | Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)	11,841
Long-term incentive plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program		$ 5.7
Average price per share for shares purchased (in dollars per share)		$ 122.40
Long-term incentive plan | Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)		46,884
Stock repurchase program
Stock repurchases
Amount of authorized stock repurchases approved for continuation under the current program			$ 150.0
Shares repurchased under stock repurchase program (in shares)	0	0
Approximate amount of repurchase authority remaining under the current stock repurchase program	$ 17.8