Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Balance at Dec. 31, 2010	$ 1,483,637	$ 1,314,199	$ 181,910	$ (12,472)
Balance (in shares) at Dec. 31, 2010		94,691,000
Increase (decrease) in stockholders' equity
Net income for common stock	28,462		28,462
Other comprehensive income (loss), net of taxes (tax benefits)	(1,838)			(1,838)
Issuance of common stock, net	15,702	15,702
Issuance of common stock, net (in shares)		598,000
Common stock dividends ($0.31 per share)	(29,412)		(29,412)
Balance at Mar. 31, 2011	1,496,551	1,329,901	180,960	(14,310)
Balance (in shares) at Mar. 31, 2011		95,289,000
Balance at Dec. 31, 2011	1,531,949	1,349,446	201,640	(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	38,316		38,316
Other comprehensive income (loss), net of taxes (tax benefits)	311			311
Issuance of common stock, net	13,434	13,434
Issuance of common stock, net (in shares)		503,000
Dividend equivalents paid on equity-classified awards	(95)		(95)
Common stock dividends ($0.31 per share)	(29,817)		(29,817)
Balance at Mar. 31, 2012	$ 1,554,098	$ 1,362,880	$ 210,044	$ (18,826)
Balance (in shares) at Mar. 31, 2012	96,541,143	96,541,000