Pension Plan Plan Assets for Qualified Pension Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Actual return on pension plan assets	11.40%
Liquidity	48.50%
Maximize Return	51.50%
Total Carrying Value	$ (596,733,000)		$ (555,400,000)
Defined Benefit Plan, Fair Value Plan Assets Before Accrued Income	594,768,000		553,651,000
Defined Benefit Plan, Fair Value of Plan Assets, Accrued Income	1,965,000		1,749,000
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(155,571,000)		(141,986,000)
Defined Benefit Plan, Fair Value Plan Assets Before Accrued Income	155,524,000		141,947,000
Defined Benefit Plan, Fair Value of Plan Assets, Accrued Income	47,000		39,000
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(441,162,000)		(413,414,000)
Defined Benefit Plan, Fair Value Plan Assets Before Accrued Income	439,244,000		411,704,000
Defined Benefit Plan, Fair Value of Plan Assets, Accrued Income	1,918,000		1,710,000
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Defined Benefit Plan, Fair Value Plan Assets Before Accrued Income	0		0
Defined Benefit Plan, Fair Value of Plan Assets, Accrued Income	0		0
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(12,063,000)		(23,668,000)
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(12,063,000)		(23,668,000)
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Future [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		(2,196,000)
Future [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Future [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		(2,196,000)
Future [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
US Government Securities and agency securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(83,915,000)	[1]	(61,592,000)	[1]
US Government Securities and agency securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0	[1]
US Government Securities and agency securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(83,915,000)		(61,592,000)	[1]
US Government Securities and agency securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	0	[1]
Domestic Corporate Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(112,811,000)	[1]	(81,601,000)
Domestic Corporate Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	0
Domestic Corporate Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(112,811,000)	[1]	(81,601,000)
Domestic Corporate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	0
Foreign Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	(10,291,000)
Foreign Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	0	[2]
Foreign Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	(10,291,000)	[2]
Foreign Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[1]	0	[2]
Other fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(2,080,000)	[2]	(3,467,000)	[2]
Other fixed income [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0	[2]
Other fixed income [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(2,080,000)		(3,467,000)	[2]
Other fixed income [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0	[2]	0	[2]
Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(93,659,000)		(71,244,000)
Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(93,659,000)		(71,244,000)
Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Mutual Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Common Trust Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(240,438,000)		(256,949,000)
Common Trust Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Common Trust Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(240,438,000)		(256,949,000)
Common Trust Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Corporate Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(49,802,000)		(47,035,000)
Corporate Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	(49,802,000)		(47,035,000)
Corporate Stock [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Corporate Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Carrying Value	0		0
Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Managers’ Securities Holdings Percentage Limit of Total Market Value of Manager's Account	7.50%
Total Carrying Value	$ (596,733,000)		$ (555,400,000)		$ (557,653,000)

[1] (a) This category represents investments in debt securities issued by the U.S. Treasury, other U.S. government corporations and agencies, states and municipalities.

[2] (b) This category primarily represents investments in commercial and residential mortgage-backed securities.