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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic EPS, Income (Numerator)
Income available to common shareholders	$ 33,853	$ 35,908	$ 37,185	$ 30,598	$ 37,497	$ 33,254	$ 35,725	$ 28,969	$ 137,544	$ 135,445	$ 126,255
Diluted EPS, Income (Numerator)
Income available to common shareholders									$ 137,544	$ 135,445	$ 126,255
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									43,299	43,020	42,744
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)									832	799	770
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding									44,131	43,819	43,514
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.78	$ 0.83	$ 0.86	$ 0.71	$ 0.87	$ 0.77	$ 0.83	$ 0.67	$ 3.18	$ 3.15	$ 2.95
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.76	$ 0.81	$ 0.84	$ 0.70	$ 0.85	$ 0.76	$ 0.82	$ 0.66	$ 3.12	$ 3.09	$ 2.90
COMPREHENSIVE EARNINGS
Tax rate used (as a percent)									35.00%	35.00%	35.00%
Other comprehensive income (loss), tax									$ (25,300)	$ 19,000	$ (3,800)