Consolidated Statements of Stockholders' Equity (Unaudited) (USD $) In Thousands	Total	Common Stock Outstanding	Common Stock	Value of Shares Held in Rabbi Trust at Cost	Deferred Compensation Obligation	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income
Beginning balance at Dec. 31, 2010	$ 436,472	$ 21,220,801	$ 210	$ (2,738)	$ 2,738	$ 226,708	$ 210,320	$ (766)
NET INCOME	22,308						22,308
OTHER COMPREHENSIVE INCOME (LOSS)	(219)							(219)
COMMON DIVIDEND DECLARED	(8,140)						(8,140)
PROCEEDS FROM EXERCISE OF STOCK OPTIONS	3,701	162,875	2			3,699
TAX BENEFIT RELATED TO EQUITY AWARD ACTIVITY	253					253
EQUITY BASED COMPENSATION	1,352					1,352
RESTRICTED STOCK AWARDS GRANTED, NET OF AWARDS	(361)	60,495				(361)
SHARES ISSUED UNDER DIRECT STOCK PURCHASE PLAN	262	10,018				262
DEFERRED COMPENSATION OBLIGATION				(119)	119
TAX BENEFIT RELATED TO DEFERRED COMPENSATION DISTRIBUTIONS	74					74
Ending balance at Jun. 30, 2011	455,702	21,454,189	212	(2,857)	2,857	231,987	224,488	(985)
Beginning balance at Dec. 31, 2011	469,057	21,499,768	213	(2,980)	2,980	233,878	239,452	(4,486)
NET INCOME	21,057						21,057
OTHER COMPREHENSIVE INCOME (LOSS)	156							156
COMMON DIVIDEND DECLARED	(9,080)						(9,080)
PROCEEDS FROM EXERCISE OF STOCK OPTIONS	459	21,658				459
TAX BENEFIT RELATED TO EQUITY AWARD ACTIVITY	59					59
EQUITY BASED COMPENSATION	1,551					1,551
RESTRICTED STOCK AWARDS GRANTED, NET OF AWARDS	(344)	85,254	1			(345)
SHARES ISSUED UNDER DIRECT STOCK PURCHASE PLAN	591	21,030				591
DEFERRED COMPENSATION OBLIGATION				(98)	98
TAX BENEFIT RELATED TO DEFERRED COMPENSATION DISTRIBUTIONS	86					86
Ending balance at Jun. 30, 2012	$ 483,592	$ 21,627,710	$ 214	$ (3,078)	$ 3,078	$ 236,279	$ 251,429	$ (4,330)