EARNINGS PER SHARE (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic Earnings Per Share:
Net earnings	$ 16,088	$ 15,557	$ 13,304	$ 36,909	$ 36,821
Less: earnings allocated to unvested restricted stock	(574)	(538)	(622)	(1,170)	(1,595)
Net earnings allocated to common shares	15,514	15,019	12,682	35,739	35,226
Weighted-average basic shares and unvested restricted stock outstanding	37,413,200	37,359,200	37,257,400	37,352,400	37,101,300
Less: weighted-average unvested restricted stock outstanding (in shares)	(1,712,800)	(1,669,200)	(1,768,900)	(1,678,800)	(1,629,800)
Weighted-average basic shares outstanding	35,700,400	35,690,000	35,488,500	35,673,600	35,471,500
Basic earnings per share (in dollars per share)	$ 0.43	$ 0.42	$ 0.36	$ 1.00	$ 0.99
Diluted Earnings Per Share:
Net earnings allocated to common shares	$ 15,514	$ 15,019	$ 12,682	$ 35,739	$ 35,226
Weighted-average basic shares outstanding	35,700,400	35,690,000	35,488,500	35,673,600	35,471,500
Diluted earnings per share (in dollars per share)	$ 0.43	$ 0.42	$ 0.36	$ 1.00	$ 0.99