Capital Stock - Stock Repurchases (Details) - Class A Common Stock - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2011
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 1,500,000	$ 3,200,000
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)	9,725	27,120
Equity compensation plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 400,000	$ 700,000
Average price per share for shares purchased (in dollars per share)	$ 132.35	$ 117.39
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)	3,246	5,830
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	$ 400,000	$ 1,100,000
Average price per share for shares purchased (in dollars per share)	$ 183.62	$ 122.19
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)	2,304	9,285
Incentive compensation deferral plan
Shares purchased outside of stock repurchase program
Total cost of shares purchased outside of publicly announced stock repurchase program	$ 700,000	$ 1,400,000
Average price per share for shares purchased (in dollars per share)	$ 175.23	$ 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 (in shares)	4,175	12,005
Stock repurchase program
Stock repurchases
Amount of authorized stock repurchases approved for continuation under the current program			$ 150,000,000
Shares repurchased under stock repurchase program (in shares)	0	0
Approximate amount of repurchase authority remaining under the current stock repurchase program	$ 17,800,000