Consolidated Statement 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	55,601		55,601
Other comprehensive income, net of tax benefits	2,629			2,629
Issuance of common stock, net	29,338	29,338
Issuance of common stock, net (in shares)		1,162,000
Common stock dividends ($0.62 per share)	(58,998)		(58,998)
Balance at Jun. 30, 2011	1,508,964	1,343,537	175,270	(9,843)
Balance (in shares) at Jun. 30, 2011		95,853,000
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	77,116		77,116
Other comprehensive income, net of tax benefits	1,861			1,861
Issuance of common stock, net	27,980	27,980
Issuance of common stock, net (in shares)		985,000
Dividend equivalents paid on equity-classified awards	(96)		(96)
Common stock dividends ($0.62 per share)	(59,791)		(59,791)
Balance at Jun. 30, 2012	$ 1,575,776	$ 1,377,426	$ 215,626	$ (17,276)
Balance (in shares) at Jun. 30, 2012	97,023,148	97,023,000