Share Repurchase Program (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands				2 Months Ended	12 Months Ended		86 Months Ended
	Feb. 07, 2018	Sep. 01, 2017	May 10, 2017	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2018
Share Repurchase Program
Increase in authorized amount		$ 1,000,000	$ 1,000,000
Authorization effective period		3 years	3 years
Remaining balance under share repurchase program					$ 715,400
Common stock repurchased, shares					9,301	5,698
Common stock repurchased, average price per share					$ 233.57	$ 264.21
Common stock repurchased, value					$ 2,172,437	$ 1,505,371
Subsequent event [Member]
Share Repurchase Program
Increase in authorized amount	$ 1,000,000
Cumulative authorized amount	$ 10,800,000
Authorization effective period	3 years
Common stock repurchased, shares				1,100			67,400
Common stock repurchased, average price per share				$ 255.48			$ 138.38
Common stock repurchased, value				$ 289,900			$ 9,300,000