Contingent Liabilities and Commitments (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 18, 2014	Dec. 31, 2014	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]
Refund payments	$ 118,000,000
Potential refund obligation		16,000,000
Site Contingency [Line Items]
Regulatory assets, total		316,900,000	294,100,000
Regulatory assets, current		77,810,000	37,520,000
Regulatory assets, noncurrent		239,080,000	256,612,000
Other Commitments [Abstract]
Commitments for construction and acquisition of property, plant, and equipment		33,000,000
Environmental assessment and remediation
Site Contingency [Line Items]
Environmental assessment and remediation costs, low estimate		4,000,000
Environmental assessment and remediation costs, high estimate		6,000,000
Accrued environmental assessment and remediation costs, total		2,700,000	4,100,000
Accrued environmental assessment and remediation costs, current		800,000	2,300,000
Accrued environmental assessment and remediation costs, noncurrent		1,900,000	1,800,000
Regulatory assets, total		1,700,000	1,800,000
Regulatory assets, current		1,200,000	1,200,000
Regulatory assets, noncurrent		500,000	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		4 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