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	38,316		38,316
Other comprehensive income (loss), net of 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	1,550,855	1,362,880	206,801	(18,826)
Balance (in shares) at Mar. 31, 2012		96,541,000
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	33,679		33,679
Other comprehensive income (loss), net of tax benefits	(61)			(61)
Issuance of common stock, net	10,216	10,216
Issuance of common stock, net (in shares)		543,000
Common stock dividends ($0.31 per share)	(30,434)		(30,434)
Balance at Mar. 31, 2013	$ 1,607,265	$ 1,413,700	$ 220,049	$ (26,484)
Balance (in shares) at Mar. 31, 2013	98,471,405	98,471,000