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	32,658						32,658
Other comprehensive income (loss)	897
Common dividend declared	(13,629)						(13,629)
Proceeds from exercise of stock options	953	48,976				953
Tax benefit related to equity award activity	153					153
Equity based compensation	2,178					2,178
Restricted stock awards issued, net of awards surrendered	(344)	85,254	1			(345)
Shares issued under direct stock purchase plan	953	32,928				953
Deferred compensation obligation				(123)	123
Tax benefit related to deferred compensation distributions	89					89
Ending balance at Sep. 30, 2012	492,965	21,666,926	214	(3,103)	3,103	237,859	258,481	(3,589)
Beginning balance at Dec. 31, 2012	529,320	22,774,009	225	(3,179)	3,179	269,950	263,671	(4,526)
Net income	39,665						39,665
Other comprehensive income (loss)	(2,534)							(2,534)
Common dividend declared	(15,128)						(15,128)
Proceeds from exercise of stock options	1,994	76,253	1			1,993
Tax benefit related to equity award activity	412					412
Equity based compensation	1,923					1,923
Restricted stock awards issued, net of awards surrendered	(669)	89,431	1			(670)
Shares issued under direct stock purchase plan	656	19,594				656
Deferred compensation obligation				(165)	165
Tax benefit related to deferred compensation distributions	105					105
Ending balance at Sep. 30, 2013	$ 555,744	$ 22,959,287	$ 227	$ (3,344)	$ 3,344	$ 274,369	$ 288,208	$ (7,060)