Consolidated Statements of Changes in Shareholders' Equity - 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	168,129		168,129
Other comprehensive income (loss), net of tax benefits	(10,628)			(10,628)
Partial settlement of equity forward	24,873	$ 24,873
Partial settlement of equity forward (in shares)		1,000,000
Issuance of common stock: Dividend reinvestment and stock purchase plan	2,461	$ 2,461
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		95,000
Issuance of common stock: Retirement savings and other plans	6,816	$ 6,816
Issuance of common stock: Retirement savings and other plans (in shares)		210,000
Issuance of common stock: Expenses and other, net	(979)	$ (979)
Common stock dividends	(126,505)		(126,505)
Ending 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
Partial settlement of equity forward	109,183	$ 109,183
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	107,460,406	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