Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common stock	Retained earnings	Accumulated other comprehensive loss
Balance at Dec. 31, 2010	$ 1,480,394	$ 1,314,199	$ 178,667	$ (12,472)
Balance (in shares) at Dec. 31, 2010		94,691,000
Increase (decrease) in stockholders' equity
Net income for common stock	104,005		104,005
Other comprehensive income, net of tax benefits	5,810			5,810
Issuance of common stock, net	33,056	33,056
Issuance of common stock, net (in shares)		1,284,000
Common stock dividends ($0.93 per share)	(88,750)		(88,750)
Balance at Sep. 30, 2011	1,534,515	1,347,255	193,922	(6,662)
Balance (in shares) at Sep. 30, 2011		95,975,000
Balance at Dec. 31, 2011	1,528,706	1,349,446	198,397	(19,137)
Balance (in shares) at Dec. 31, 2011	96,038,328	96,038,000
Increase (decrease) in stockholders' equity
Net income for common stock	124,822		124,822
Other comprehensive income, net of tax benefits	3,447			3,447
Issuance of common stock, net	40,161	40,161
Issuance of common stock, net (in shares)		1,387,000
Dividend equivalents paid on equity-classified awards	(99)		(99)
Common stock dividends ($0.93 per share)	(89,902)		(89,902)
Balance at Sep. 30, 2012	$ 1,607,135	$ 1,389,607	$ 233,218	$ (15,690)
Balance (in shares) at Sep. 30, 2012	97,425,467	97,425,000