CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Common stock, par value (in usd per share)	$ 0.001	$ 0.001	$ 0.001
Options exercised, shares	208,306	142,203	85,532
Conversion of common stock units, shares	15,981	15,629	8,293
Conversion of restricted stock units, shares	313,292	222,297	198,681
Cash dividends (in usd per share)	$ 1.10	$ 1.06	$ 1.00
Treasury stock, shares	24,721,000	24,672,932	23,971,522
Treasury stock, acquisition of shares	48,440	701,410	663,092