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, 2012	$ 1,593,865	$ 1,403,484	$ 216,804	$ (26,423)
Balance (in shares) at Dec. 31, 2012		97,928,000
Increase (decrease) in stockholders' equity
Net income for common stock	33,679		33,679
Other comprehensive income, net of taxes	(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		98,471,000
Balance at Dec. 31, 2013	1,727,070	1,488,126	255,694	(16,750)
Balance (in shares) at Dec. 31, 2013	101,259,800	101,260,000
Increase (decrease) in stockholders' equity
Net income for common stock	45,927		45,927
Other comprehensive income, net of taxes	1,167			1,167
Issuance of common stock, net	3,212	3,212
Issuance of common stock, net (in shares)		218,000
Common stock dividends ($0.31 per share)	(31,448)		(31,448)
Balance at Mar. 31, 2014	$ 1,745,928	$ 1,491,338	$ 270,173	$ (15,583)
Balance (in shares) at Mar. 31, 2014	101,477,616	101,478,000