STOCKHOLDERS' EQUITY - Common Stock (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity
Shares outstanding	48,102,328	47,828,079	47,487,510
Purchase of common stock	(17,445)	(123,577)	(260,088)
Shares issued under stock incentive plan, net of awards surrender to pay applicable taxes	385,037	192,050	218,458
Exercise of stock options	22,500	202,500	377,671
Shares issued to Board of Directors in lieu of cash	5,522	3,276	4,528
Shares outstanding	48,497,942	48,102,328	47,828,079