EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Earning per share
Authorized common stock (in shares)	199,000								199,000
Common stock, par value (in dollars per share)	$ 0.01								$ 0.01			$ 0.01
Income (Numerator)
Income available to common stockholders, basic (in dollars)	$ 49,513	$ 53,523	$ 65,174	$ 51,820	$ 49,581	$ 53,098	$ 59,048	$ 43,863	$ 220,030	$ 205,590	$ 199,348
Income available to common stockholders, diluted (in dollars)									$ 220,030	$ 205,590	$ 199,348
Shares (Denominator)
Basic (in shares)	61,944	62,592	62,631	62,355	62,586	62,858	63,053	62,722	62,435	62,804	62,585
Total average equivalent shares	64,528	64,821	64,828	64,234	64,220	64,690	64,785	65,063	64,596	64,849	64,492
Per Share Amount
Net income per share, basic (in dollars per share)	$ 0.80	$ 0.86	$ 1.04	$ 0.83	$ 0.79	$ 0.84	$ 0.94	$ 0.70	$ 3.52	$ 3.27	$ 3.19
Net income per share, diluted (in dollars per share)	$ 0.77	$ 0.83	$ 1.01	$ 0.81	$ 0.77	$ 0.82	$ 0.91	$ 0.67	$ 3.41	$ 3.17	$ 3.09
Stock options
Shares (Denominator)
Effect of dilutive stock based compensation (in shares)									2,106	1,988	1,864
Restricted stock units
Shares (Denominator)
Effect of dilutive stock based compensation (in shares)									55	57	43