CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - $ / shares	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Common stock, par value (in usd per share)	$ 0.001		$ 0.001
Treasury stock (in shares)	24,721,372		24,672,932
Common stock
Common stock, par value (in usd per share)			$ 0.001	$ 0.001
Options exercised (in shares)	156,211	114,507
Conversion of common stock units (in shares)	15,981	15,629
Conversion of restricted stock units (in shares)	293,002	188,207
Retained earnings
Cash dividends (in usd per share)	$ 0.825	$ 0.795
Treasury stock, at cost
Treasury stock (in shares)	24,721,372	24,672,932	24,672,932	23,971,522
Treasury stock, acquisitions (in shares)	48,440	701,410