Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common stock	Retained earnings	AOCI
Beginning Balance at Dec. 31, 2015	$ 1,927,640	$ 1,629,136	$ 324,766	$ (26,262)
Balance (in shares) at Dec. 31, 2015		107,460,000
Increase (decrease) in stockholders' equity
Net income for common stock	248,256		248,256
Other comprehensive income (loss), net of tax benefits	(6,867)			(6,867)
Issuance of common stock: Dividend reinvestment and stock purchase plan	26,844	$ 26,844
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		859,000
Issuance of common stock: Retirement savings and other plans	9,298	$ 9,298
Issuance of common stock: Retirement savings and other plans (in shares)		264,000
Issuance of common stock: Expenses and other, net	(4,368)	$ (4,368)
Common stock dividends	(134,050)		(134,050)
Ending Balance at Dec. 31, 2016	2,066,753	$ 1,660,910	438,972	(33,129)
Balance (in shares) at Dec. 31, 2016		108,583,000
Increase (decrease) in stockholders' equity
Net income for common stock	165,297		165,297
Other comprehensive income (loss), net of tax benefits	(1,372)			(1,372)
Reclass of AOCI for tax rate reduction impact	0		7,440	(7,440)
Issuance of common stock: Retirement savings and other plans	4,664	$ 4,664
Issuance of common stock: Retirement savings and other plans (in shares)		205,000
Issuance of common stock: Expenses and other, net	(3,083)	$ (3,083)
Common stock dividends	(134,873)		(134,873)
Ending Balance at Dec. 31, 2017	$ 2,097,386	$ 1,662,491	476,836	(41,941)
Balance (in shares) at Dec. 31, 2017	108,787,807	108,788,000
Increase (decrease) in stockholders' equity
Net income for common stock	$ 201,774		201,774
Other comprehensive income (loss), net of tax benefits	(8,669)			(8,669)
Issuance of common stock: Retirement savings and other plans	2,650	$ 2,650
Issuance of common stock: Retirement savings and other plans (in shares)		91,000
Issuance of common stock: Expenses and other, net	4,126	$ 4,126
Common stock dividends	(134,987)		(134,987)
Ending Balance at Dec. 31, 2018	$ 2,162,280	$ 1,669,267	$ 543,623	$ (50,610)
Balance (in shares) at Dec. 31, 2018	108,879,245	108,879,000