Consolidated Statements of Changes in Equity and Comprehensive Income (Parenthetical) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Statement of Stockholders' Equity [Abstract]
Dividends Declared per Common Share (in USD per share)		$ 1.02		$ 0.96		$ 0.9
Combined statutory tax rate		40.00%		40.00%		41.00%
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[1]	$ (30,258)
Changes During Year	[2]	5,759		$ (9,498)		$ 10,345
Balance at December 31	[1]	(24,499)		(30,258)
Postretirement Liability Adjustment [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[2]	(27,663)	[1]	(18,503)		(23,437)
Changes During Year	[2]	5,518		(9,160)		4,934
Balance at December 31	[2]	(22,145)	[1]	(27,663)	[1]	(18,503)
Unrealized Gain (Loss) on Derivatives-Other [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[2]	(2,450)	[1]	(2,697)		(2,962)
Changes During Year	[2]	321		247		265
Balance at December 31	[2]	(2,129)	[1]	(2,450)	[1]	(2,697)
Unrealized Gain (Loss) on Available-for-Sale Securities [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[2]	(75)	[1]	397		294
Changes During Year	[2]	(53)		(472)		103
Balance at December 31	[2]	(128)	[1]	(75)	[1]	397
Other Comprehensive Income (Loss) of Affiliated Companies [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[2]	(70)		43		(5,000)
Changes During Year	[2]	(27)		(113)		5,043
Balance at December 31	[2]	(97)		(70)		43
Accumulated Other Comprehensive Loss [Member]
Change In Accumulated Other Comprehensive Loss Balances [Roll Forward]
Balance at January 1	[2]	(30,258)		(20,760)		(31,105)
Changes During Year	[2]	5,759		(9,498)		10,345
Balance at December 31	[2]	$ (24,499)		$ (30,258)		$ (20,760)

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

[2] For 2015 and 2014, determined using a combined average statutory tax rate of 40%. For 2013, determined using a combined statutory tax rate of 41%.