Consolidated Statements of Changes in Equity and Comprehensive Income - USD ($) $ in Thousands		Total		Common Stock	Premium on Common Stock	Treasury Stock	Accumulated Other Comprehensive Loss		Retained Earnings	Postretirement Liability Adjustment		Unrealized Gain (Loss) on Derivatives-Other		Unrealized Gain (Loss) on Available- for-Sale Securities		Other Comprehensive Income (Loss) of Affiliated Companies
Balance at Dec. 31, 2013		$ 827,000		$ 81,788	$ 360,303	$ (372)	$ (20,760)		$ 406,041
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		97,046							97,046
Common Stock Issued or Granted Through Equity Offering or Stock Plans		81,753		3,630	78,081	42
Cash Dividends Declared - Common Stock		(63,869)							(63,869)
Balance at Dec. 31, 2014		932,432		85,418	438,384	(330)	(30,258)		439,218
Balance at January 1 at Dec. 31, 2013	[1]						(20,760)			$ (18,503)		$ (2,697)		$ 397		$ 43
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Changes During Year		(9,498)	[2],[3]				(9,498)	[1],[3]		(9,160)	[1]	247	[1]	(472)	[1]	(113)	[1]
Balance at December 31 at Dec. 31, 2014	[1]						(30,258)			(27,663)		(2,450)		(75)		(70)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		105,107							105,107
Common Stock Issued or Granted Through Equity Offering or Stock Plans		64,399		3,289	61,076	34
Cash Dividends Declared - Common Stock		(70,158)							(70,158)
Balance at Dec. 31, 2015		1,037,539		88,707	499,460	(296)	(24,499)		474,167
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Changes During Year		5,759	[2],[3]				5,759	[1],[3]		5,518	[1]	321	[1]	(53)	[1]	(27)	[1]
Balance at December 31 at Dec. 31, 2015		(24,499)					(24,499)	[1]		(22,145)	[1]	(2,129)	[1]	(128)	[1]	(97)	[1]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		118,810							118,810
Common Stock Issued or Granted Through Equity Offering or Stock Plans		218,153		10,640	207,483	30
Cash Dividends Declared - Common Stock		(82,380)							(82,380)
Balance at Dec. 31, 2016		$ 1,289,240		$ 99,347	$ 706,943	$ (266)	(27,381)		$ 510,597
Additional Statement Information [Abstract]
Combined average statutory tax rate		40.00%
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Changes During Year		$ (2,882)	[2],[3]				(2,882)	[1],[3]		(3,197)	[1]	197	[1]	118	[1]	0	[1]
Balance at December 31 at Dec. 31, 2016		$ (27,381)					$ (27,381)	[1]		$ (25,342)	[1]	$ (1,932)	[1]	$ (10)	[1]	$ (97)	[1]

[1] Determined using a combined average statutory tax rate of 40%.

[2] Determined using a combined average statutory tax rate of 40%.

[3] Determined using a combined average statutory tax rate of 40%.