Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common stock	Retained earnings	AOCI
Beginning Balance at Dec. 31, 2014	$ 1,790,573	$ 1,521,297	$ 296,654	$ (27,378)
Balance (in shares) at Dec. 31, 2014		102,565,000
Increase (decrease) in stockholders' equity
Net income for common stock	159,877		159,877
Other comprehensive income (loss), net of tax benefits	1,116			1,116
Issuance of common stock: Partial settlement of equity forward	109,183	$ 109,183
Issuance of common stock: Partial settlement of equity forward (in shares)		4,700,000
Issuance of common stock: Retirement savings and other plans	5,578	$ 5,578
Issuance of common stock: Retirement savings and other plans (in shares)		195,000
Issuance of common stock: Expenses and other, net	(6,922)	$ (6,922)
Common stock dividends	(131,765)		(131,765)
Ending 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	108,583,413	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