Share Repurchase Program and Treasury Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Share Repurchase Program and Treasury Stock (Textuals) [Abstract]
Remaining Amount to Repurchase under the program	$ 345,900
Shares Purchased Under Share Repurchase Program	27,100,000
Treasury Stock, Value, Acquired, Cost Method	$ 237,499	$ 249,900	$ 399,997
Number of shares repurchased	(396,218)	(505,593)
Average price of share repurchased, per share	$ 599.40	$ 494.35
Exercise of stock options and restricted stock units, shares reissued	68,653	153,413