Consolidated Statements of Stockholders' Equity (Parentheticals) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Weighted average price per treasury share withheld for taxes (per share)	$ 48.31	$ 44.27	$ 46.83