Capital Stock (Details 2) - Class A - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2011
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 7.3	$ 1.5
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)	60,332	15,093
Equity compensation plan (ECP)
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 0.4
Average price per share for shares purchased (in dollars per share)	$ 111.55
Equity compensation plan (ECP) | 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)	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	$ 1.2	$ 0.7	$ 9.0
Average price per share for shares purchased (in dollars per share)	$ 121.85	$ 99.23	$ 94.99
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)	9,663	7,432	94,938
Long-term incentive plan (LTIP)
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 5.7	$ 0.8
Average price per share for shares purchased (in dollars per share)	$ 122.40	$ 98.20
Long-term incentive plan (LTIP) | 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	7,661
Stock repurchase program
Stock repurchases
Shares repurchased under stock repurchase program (in shares)	0	0
Amount of authorized stock repurchases approved for continuation under the current program				$ 150.0
Approximate amount of repurchase authority remaining under the current stock repurchase program	$ 17.8