Stockholders' Equity (Schedule of Restricted Stock Grants) (Details) - Restricted Stock - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award
Granted	$ 14,018	$ 17,439	$ 20,558
Weighted average stock price	$ 92.25	$ 74.55	$ 59.32
Fair value of stock granted	1,293	1,300	1,220