Contingent Liabilities and Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Site Contingency [Line Items]
Regulatory assets, total	$ 294,100,000	$ 251,600,000
Regulatory assets, current	37,520,000	36,706,000
Regulatory assets, noncurrent	256,612,000	214,912,000
Other Commitments [Abstract]
Commitments for construction and acquisition of property, plant, and equipment	368,000,000
Environmental assessment and remediation
Site Contingency [Line Items]
Environmental assessment and remediation costs, low estimate	6,000,000
Environmental assessment and remediation costs, high estimate	8,000,000
Accrued environmental assessment and remediation costs, total	4,100,000	3,300,000
Accrued environmental assessment and remediation costs, current	2,300,000	1,100,000
Accrued environmental assessment and remediation costs, noncurrent	1,800,000	2,200,000
Regulatory assets, total	1,800,000	0
Regulatory assets, current	1,200,000
Regulatory assets, noncurrent	600,000
Environmental assessment and remediation | Minimum
Site Contingency [Line Items]
Expected duration of environmental assessment and remediation spending	3 years
Environmental assessment and remediation | Maximum
Site Contingency [Line Items]
Expected duration of environmental assessment and remediation spending	5 years
Potentially responsible party at various Superfund and state waste disposal sites
Site Contingency [Line Items]
Environmental assessment and remediation costs, high estimate	$ 500,000