Consolidated Statements of Changes in Shareholders' Equity (unaudited) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common stock	Retained Earnings	Accumulated other comprehensive income (loss)
Balance at Dec. 31, 2013	$ 1,726,406	$ 1,488,126	$ 255,030	$ (16,750)
Balance (in shares) at Dec. 31, 2013		101,260,000
Increase (decrease) in stockholders' equity
Net income for common stock	45,787		45,787
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	1,745,124	1,491,338	269,369	(15,583)
Balance (in shares) at Mar. 31, 2014		101,478,000
Balance at Dec. 31, 2014	1,790,573	1,521,297	296,654	(27,378)
Balance (in shares) at Dec. 31, 2014	102,565,266	102,565,000
Increase (decrease) in stockholders' equity
Net income for common stock	31,866		31,866
Other comprehensive income, net of taxes	4,058			4,058
Issuance of common stock, net	103,252	103,252
Issuance of common stock, net (in shares)		4,853,000
Common stock dividends ($0.31 per share)	(31,840)		(31,840)
Balance at Mar. 31, 2015	$ 1,897,909	$ 1,624,549	$ 296,680	$ (23,320)
Balance (in shares) at Mar. 31, 2015	107,417,644	107,418,000