Condensed 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, 2015	$ 1,927,640	$ 1,629,136	$ 324,766	$ (26,262)
Beginning Balance (in shares) at Dec. 31, 2015		107,460,000
Increase (decrease) in stockholders' equity
Net income for common stock	32,352		32,352
Other comprehensive income, net of taxes	8,800			8,800
Issuance of common stock, net of expenses	6,754	$ 6,754
Issuance of common stock, net of expenses (in shares)		416,000
Common stock dividends	(33,367)		(33,367)
Ending Balance at Mar. 31, 2016	1,942,179	$ 1,635,890	323,751	(17,462)
Ending Balance (in shares) at Mar. 31, 2016		107,876,000
Beginning Balance at Dec. 31, 2016	$ 2,066,753	$ 1,660,910	438,972	(33,129)
Beginning Balance (in shares) at Dec. 31, 2016	108,583,413	108,583,000
Increase (decrease) in stockholders' equity
Net income for common stock	$ 34,193		34,193
Other comprehensive income, net of taxes	985			985
Issuance of common stock, net of expenses	(2,630)	$ (2,630)
Issuance of common stock, net of expenses (in shares)		162,000
Common stock dividends	(33,713)		(33,713)
Ending Balance at Mar. 31, 2017	$ 2,065,588	$ 1,658,280	$ 439,452	$ (32,144)
Ending Balance (in shares) at Mar. 31, 2017	108,745,265	108,745,000