CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - $ / shares	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Common stock, par value (in usd per share)	$ 0.001		$ 0.001
Treasury stock (in shares)	24,672,932		24,672,932
Common stock
Common stock, par value (in usd per share)			$ 0.001	$ 0.001
Options exercised (in shares)	127,774	94,225
Conversion of common stock units (in shares)	15,981	8,538
Conversion of restricted stock units (in shares)	280,341	171,042
Retained earnings
Cash dividends (in usd per share)	$ 0.55	$ 0.53
Treasury stock, at cost
Treasury stock (in shares)	24,672,932	24,672,932	24,672,932	23,971,522
Treasury stock, acquisitions (in shares)	0	701,410