Share Repurchase Program and Treasury Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Share Repurchase Program and Treasury Stock (Textuals) [Abstract]
Authorized amount under share repurchase program	$ 464,700
Shares Purchased Under Share Repurchase Program	26,900,000
Repurchases of common stock	$ (118,750)		$ (399,997)
Number of shares repurchased	(187,880)	(275,088)
Average price of share repurchased, per share	$ 632.03	$ 454.37
Exercise of stock options and restricted stock units, shares reissued	39,362	76,849