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, 2011	$ 1,528,706	$ 1,349,446	$ 198,397	$ (19,137)
Balance (in shares) at Dec. 31, 2011		96,038,000
Increase (decrease) in stockholders' equity
Net income for common stock	77,116		77,116
Other comprehensive loss, 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		97,023,000
Balance at Dec. 31, 2012	1,593,865	1,403,484	216,804	(26,423)
Balance (in shares) at Dec. 31, 2012	97,928,403	97,928,000
Increase (decrease) in stockholders' equity
Net income for common stock	74,267		74,267
Other comprehensive loss, net of tax benefits	(8,366)			(8,366)
Issuance of common stock, net	25,887	25,887
Issuance of common stock, net (in shares)		1,116,000
Common stock dividends ($0.62 per share)	(61,004)		(61,004)
Balance at Jun. 30, 2013	$ 1,624,649	$ 1,429,371	$ 230,067	$ (34,789)
Balance (in shares) at Jun. 30, 2013	99,044,053	99,044,000