Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	2 Months Ended	12 Months Ended		86 Months Ended
	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2018
Earnings Per Share
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]
Earnings Per Share
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