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Stockholders' Equity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2017	May 31, 2015
Stockholders' equity
Common stock, shares authorized (in shares)	153,000,000	153,000,000
Common Stock
Share repurchase activity
Shares repurchased (in shares)	2,400,000	200,000	1,700,000
Average price (in usd per share)	$ 173.19	$ 161.16	$ 209.39
Common Stock | January 2017 Plan
Stockholders' equity
Stock repurchase program, number of shares authorized (in shares)				1,900,000
Share repurchase activity
Stock repurchase plan, number of remaining shares to be repurchased (in shares)	1,600,000
Common Stock | May 2015 Plan
Stockholders' equity
Stock repurchase program, number of shares authorized (in shares)					3,000,000
Share repurchase activity
Stock repurchase plan, number of remaining shares to be repurchased (in shares)	0
Voting Common Stock
Stockholders' equity
Common stock, shares authorized (in shares)	150,000,000.0
Class B Non-Voting Common Stock
Stockholders' equity
Common stock, shares authorized (in shares)	3,000,000.0
Preferred Stock
Share repurchase activity
Preferred stock, shares authorized (in shares)	5,000,000.0

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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