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	124,822		124,822
Other comprehensive loss, 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		97,425,000
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	122,503		122,503
Other comprehensive loss, net of tax benefits	(9,263)			(9,263)
Issuance of common stock, net	40,099	40,099
Issuance of common stock, net (in shares)		1,614,000
Common stock dividends ($0.93 per share)	(91,739)		(91,739)
Balance at Sep. 30, 2013	$ 1,655,465	$ 1,443,583	$ 247,568	$ (35,686)
Balance (in shares) at Sep. 30, 2013	99,541,518	99,542,000