Consolidated Statements Of Stockholders Equity (USD $) In Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income Loss [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2010	$ 1,066,124	$ 1,143	$ 1,094,713	$ 6,564	$ (33,732)	$ (2,564)
Net income	39,537			39,537
Change in other comprehensive income, net of tax	3,016				3,016
Common stock dividends	(29,753)			(29,753)
Issuance of common stock	122,456	124	123,180			(848)
Restricted stock compensation	1,832		1,832
Tax expense of stock-based compensation	(62)		(62)
Balance at Jun. 30, 2011	1,203,150	1,267	1,219,663	16,348	(30,716)	(3,412)
Balance at Dec. 31, 2011	1,210,199	1,268	1,224,572	32,925	(45,148)	(3,418)
Net income	50,712			50,712
Change in other comprehensive income, net of tax	3,787				3,787
Common stock dividends	(33,775)			(33,775)
Issuance of common stock	138,994	128	140,704	(377)		(1,461)
Restricted stock compensation	2,206		2,206
Tax expense of stock-based compensation	373		373
Balance at Jun. 30, 2012	$ 1,372,496	$ 1,396	$ 1,367,855	$ 49,485	$ (41,361)	$ (4,879)