Consolidated Statements of Changes in Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common stock	Retained Earnings	Accumulated other comprehensive income (loss)
Beginning 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	134,876		134,876
Other comprehensive income, 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	(94,711)		(94,711)
Ending Balance at Sep. 30, 2014	1,800,457	$ 1,519,256	295,195	(13,994)
Balance (in shares) at Sep. 30, 2014		102,562,000
Beginning 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	$ 117,557		117,557
Other comprehensive income, net of taxes	5,361			5,361
Issuance of common stock, net	105,962	$ 105,962
Issuance of common stock, net (in shares)		4,894,000
Common stock dividends	(98,452)		(98,452)
Ending Balance at Sep. 30, 2015	$ 1,921,001	$ 1,627,259	$ 315,759	$ (22,017)
Balance (in shares) at Sep. 30, 2015	107,458,641	107,459,000