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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic EPS, Income (Numerator)
Income available to common shareholders									$ 64,179	$ 105,028	$ 114,920
Diluted EPS, Income (Numerator)
Income available to common shareholders									$ 64,179	$ 105,028	$ 114,920
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,358	44,033	43,772
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)									477	467	660
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,835	44,500	44,432
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ (0.46)	$ 0.89	$ 0.75	$ 0.28	$ 1.30	$ 0.04	$ 0.60	$ 0.45	$ 1.45	$ 2.39	$ 2.63
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ (0.46)	$ 0.88	$ 0.74	$ 0.27	$ 1.29	$ 0.04	$ 0.59	$ 0.45	$ 1.43	$ 2.36	$ 2.59