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	122,503		122,503
Other comprehensive income (loss), net of taxes	(9,263)			(9,263)
Issuance of common stock, net	40,099	40,099
Issuance of common stock, net (in shares)		1,614,000
Common stock dividends ($0.93 per share)	(91,739)		(91,739)
Balance at Sep. 30, 2013	1,655,465	1,443,583	247,568	(35,686)
Balance (in shares) at Sep. 30, 2013		99,542,000
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	135,163		135,163
Other comprehensive income (loss), net of taxes	2,756			2,756
Issuance of common stock, net	31,130	31,130
Issuance of common stock, net (in shares)		1,302,000
Common stock dividends ($0.93 per share)	(94,711)		(94,711)
Balance at Sep. 30, 2014	$ 1,801,408	$ 1,519,256	$ 296,146	$ (13,994)
Balance (in shares) at Sep. 30, 2014	102,562,464	102,562,000