Benefit Plans (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Regulatory Liability, Noncurrent	$ 232,888,000	$ 182,848,000
Regulatory Assets, Noncurrent	214,912,000	207,945,000
RegulatoryNoncurrentAssetAmortizationPeriod	10
AllocatedShareBasedCompensationExpense	2,800,000	2,400,000	3,100,000
Noncontributory defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Cost	20,300,000	16,400,000	16,700,000
OtherPostretirementBenefitPlansDefinedBenefitMember
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Cost	2,500,000	3,000,000	4,500,000
Regulatory Liability, Noncurrent	24,700,000	22,000,000
Regulatory Assets, Noncurrent	4,600,000	5,700,000
DefinedContributionPensionMember
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Cost	$ 7,100,000	$ 6,800,000	$ 6,700,000