Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended		88 Months Ended
	May 07, 2018	Mar. 31, 2018	Mar. 31, 2017	May 07, 2018
Earnings Per Share
Common stock repurchased, shares		2,188	1,829
Common stock repurchased, average price per share		$ 251.08	$ 268.09
Common stock repurchased, value		$ 549,428	$ 490,312
Subsequent event [Member]
Earnings Per Share
Common stock repurchased, shares	500			68,900
Common stock repurchased, average price per share	$ 238.90			$ 140.75
Common stock repurchased, value	$ 117,500			$ 9,700,000