Capital Stock (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Capital Stock [Abstract]
Amount of common stock Board of Directors authorized to be repurchased after previous authorizations, maximum (in thousands of dollars)	$ 500,000
Number of shares repurchased (in shares) | shares	1,584
Average price per share for the shares repurchased (in dollars per share) | $ / shares	$ 94.74
Purchases of common stock	$ (150,054)
Amount remaining under the authorization	$ 177,966