EARNINGS PER SHARE Schedule of Basic and Diluted Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Numerator:
NET INCOME	$ 58,984		$ 51,183		$ 156,569		$ 139,620
Dividends declared	(22,299)		(19,446)		(61,820)		(55,684)
Undistributed earnings	36,685		31,737		94,749		83,936
Percentage allocated to common shares	99.10%	[1]	98.70%	[1]	99.00%	[1]	98.70%	[1]
Numerator for basic and diluted earnings per common share	58,453		50,537		155,042		137,816
Denominator:
Denominator for basic earnings per common share — weighted average common shares outstanding	51,986,293		50,863,727		51,875,787		50,748,257
Incremental shares for stock options and employee stock purchase plan	435,798		745,972		417,144		754,437
Denominator for diluted earnings per common share — adjusted weighted average shares and assumed conversion	52,422,091		51,609,699		52,292,931		51,502,694
Per common share net income:
Basic	$ 1.12		$ 0.99		$ 2.99		$ 2.72
Diluted	$ 1.12		$ 0.98		$ 2.96		$ 2.68
Percentage allocated to common shares:
Weighted average restricted shares (participating securities)	476,177		644,233		499,573		677,070
Denominator for percentage allocated to common shares - total weighted-average shares outstanding	52,462,470		51,507,960		52,375,360		51,425,327
Common Shares
Numerator:
Dividends declared	22,098		19,213		61,240		54,971
Undistributed earnings	$ 36,355		$ 31,324		$ 93,802		$ 82,845

[1] Weighted average common shares outstanding51,986,293 50,863,727 51,875,787 50,748,257Weighted average restricted shares (participating securities)476,177 644,233 499,573 677,070 Total52,462,470 51,507,960 52,375,360 51,425,327 Percentage allocated to common shares99.1% 98.7% 99.0% 98.7%