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, 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	74,267		74,267
Other comprehensive income (loss), net of taxes	(8,366)			(8,366)
Issuance of common stock, net	25,887	25,887
Issuance of common stock, net (in shares)		1,116,000
Common stock dividends ($0.62 per share)	(61,004)		(61,004)
Balance at Jun. 30, 2013	1,624,649	1,429,371	230,067	(34,789)
Balance (in shares) at Jun. 30, 2013		99,044,000
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	87,348		87,348
Other comprehensive income (loss), net of taxes	4,067			4,067
Issuance of common stock, net	5,310	5,310
Issuance of common stock, net (in shares)		300,000
Common stock dividends ($0.62 per share)	(62,916)		(62,916)
Balance at Jun. 30, 2014	$ 1,760,879	$ 1,493,436	$ 280,126	$ (12,683)
Balance (in shares) at Jun. 30, 2014	101,560,176	101,560,000