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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic EPS, Income (Numerator)
Income available to common shareholders	$ 32,187	$ 22,263	$ 29,077	$ 31,393	$ 33,853	$ 35,908	$ 37,185	$ 30,598	$ 114,920	$ 137,544	$ 135,445
Diluted EPS, Income (Numerator)
Income available to common shareholders									$ 114,920	$ 137,544	$ 135,445
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									43,772	43,299	43,020
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)									660	832	799
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,432	44,131	43,819
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.73	$ 0.51	$ 0.67	$ 0.72	$ 0.78	$ 0.83	$ 0.86	$ 0.71	$ 2.63	$ 3.18	$ 3.15
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.72	$ 0.50	$ 0.65	$ 0.71	$ 0.76	$ 0.81	$ 0.84	$ 0.70	$ 2.59	$ 3.12	$ 3.09