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
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 issued, net of awards surrendered	(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)
Beginning balance at Dec. 31, 2012	529,320	22,774,009	225	(3,179)	3,179	269,950	263,671	(4,526)
Net income	25,010						25,010
Other comprehensive income (loss)	(2,871)							(2,871)
Common dividend declared	(10,070)						(10,070)
Proceeds from exercise of stock options	744	30,029				744
Tax benefit related to equity award activity	312					312
Equity based compensation	1,389					1,389
Restricted stock awards issued, net of awards surrendered	(668)	89,431	1			(669)
Shares issued under direct stock purchase plan	339	10,815				339
Deferred compensation obligation				(115)	115
Tax benefit related to deferred compensation distributions	100					100
Ending balance at Jun. 30, 2013	$ 543,605	$ 22,904,284	$ 226	$ (3,294)	$ 3,294	$ 272,165	$ 278,611	$ (7,397)