Share Repurchase Program (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		1 Months Ended	3 Months Ended		88 Months Ended
	Feb. 07, 2018	May 07, 2018	Mar. 31, 2018	Mar. 31, 2017	May 07, 2018
Share Repurchase Program
Increase in authorized amount	$ 1,000,000
Cumulative authorized amount	$ 10,800,000
Authorization effective period	3 years
Remaining balance under share repurchase program			$ 1,200,000
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]
Share Repurchase Program
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