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, 2012	$ 1,593,008	$ 1,403,484	$ 215,947	$ (26,423)
Balance (in shares) at Dec. 31, 2012		97,928,000
Increase (decrease) in stockholders' equity
Net income for common stock	161,709		161,709
Other comprehensive income (loss), net of tax benefits	9,673			9,673
Partial settlement of equity forward	33,409	$ 33,409
Partial settlement of equity forward (in shares)		1,300,000
Issuance of common stock: Dividend reinvestment and stock purchase plan	41,692	$ 41,692
Issuance of common stock: Dividend reinvestment and stock purchase plan (in shares)		1,612,000
Issuance of common stock: Retirement savings and other plans	9,203	$ 9,203
Issuance of common stock: Retirement savings and other plans (in shares)		420,000
Issuance of common stock: Expenses and other, net	338	$ 338
Common stock dividends	(122,626)		(122,626)
Ending 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	102,565,266	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