Capital Stock - Stock Repurchases (Details) - Class A Common Stock - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2011
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 3,200,000	$ 7,300,000	$ 1,500,000
Performance shares
Shares purchased outside of stock repurchase program
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards	27,120	60,332	15,093
Equity compensation plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 700,000	$ 400,000
Average price per share for shares purchased	$ 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	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	$ 1,100,000	$ 1,200,000	$ 700,000
Average price per share for shares purchased	$ 122.19	$ 121.85	$ 99.23
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	9,285	9,663	7,432
Incentive compensation deferral plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 1,400,000
Average price per share for shares purchased	$ 119.28
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	12,005
Long-term incentive plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program		$ 5,700,000	$ 800,000
Average price per share for shares purchased		$ 122.40	$ 98.20
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		46,884	7,661
Stock repurchase program
Stock repurchases
Shares repurchased under stock repurchase program (in shares)	0	0	0
Amount of authorized stock repurchases approved for continuation under the current program				$ 150,000,000
Approximate amount of repurchase authority remaining under the current stock repurchase program	$ 17,800,000