SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - 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
Basic EPS, Income (Numerator)
Income available to common shareholders	$ 57,258	$ 1,734	$ 26,208	$ 19,828	$ 32,187	$ 22,263	$ 29,077	$ 31,393	$ 105,028	$ 114,920	$ 137,544
Diluted EPS, Income (Numerator)
Income available to common shareholders									$ 105,028	$ 114,920	$ 137,544
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,033	43,772	43,299
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)									467	660	832
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,500	44,432	44,131
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 1.30	$ 0.04	$ 0.60	$ 0.45	$ 0.73	$ 0.51	$ 0.67	$ 0.72	$ 2.39	$ 2.63	$ 3.18
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 1.29	$ 0.04	$ 0.59	$ 0.45	$ 0.72	$ 0.50	$ 0.65	$ 0.71	$ 2.36	$ 2.59	$ 3.12