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	81,455			81,455
Change in other comprehensive income, net of tax	6,176				6,176
Common stock dividends ($0.36/share)	(50,705)			(50,705)
Issuance of common stock	144,037	129	145,833	(377)		(1,548)
Restricted stock compensation	3,451		3,451
Tax expense of stock-based compensation	385		385
Balance at Sep. 30, 2012	1,394,998	1,397	1,374,241	63,298	(38,972)	(4,966)
Balance at Dec. 31, 2012	1,402,069	1,398	1,376,601	75,312	(46,224)	(5,018)
Net income	89,365			89,365
Change in other comprehensive income, net of tax	(19,947)				(19,947)
Common stock dividends ($0.36/share)	(52,028)			(52,028)
Issuance of common stock	57,571	57	59,561			(2,047)
Restricted stock compensation	3,339		3,339
Tax expense of stock-based compensation	1,278		1,278
Balance at Sep. 30, 2013	$ 1,481,647	$ 1,455	$ 1,440,779	$ 112,649	$ (66,171)	$ (7,065)