Consolidated Statements Of Stockholders' Equity (USD $) In Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2011	$ 1,210,199	$ 1,268	$ 1,224,572	$ 32,925	$ (45,148)	$ (3,418)
Net income	21,582			21,582
Change in other comprehensive income, net of tax	1,413				1,413
Common stock dividends	(16,858)			(16,858)
Issuance of common stock	137,031	125	138,052	(377)		(769)
Restricted stock compensation	998		998
Tax expense of stock-based compensation	334		334
Balance at Mar. 31, 2012	1,354,699	1,393	1,363,956	37,272	(43,735)	(4,187)
Balance at Dec. 31, 2012	1,402,069	1,398	1,376,601	75,312	(46,224)	(5,018)
Net income	28,538			28,538
Change in other comprehensive income, net of tax	(974)				(974)
Common stock dividends	(16,927)			(16,927)
Issuance of common stock	(1,779)	8	155			(1,942)
Restricted stock compensation	1,083		1,083
Tax expense of stock-based compensation	1,247		1,247
Balance at Mar. 31, 2013	$ 1,413,257	$ 1,406	$ 1,379,086	$ 86,923	$ (47,198)	$ (6,960)