Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Potential refund obligation	$ 17,800
Site Contingency [Line Items]
Regulatory assets, total	343,300	$ 316,900
Regulatory assets, current	79,575	77,810
Regulatory assets, noncurrent	263,730	239,080
Other Commitments [Abstract]
Commitments for construction and acquisition of property, plant, and equipment	417,000
Environmental assessment and remediation
Site Contingency [Line Items]
Environmental assessment and remediation costs, low estimate	4,000
Environmental assessment and remediation costs, high estimate	6,000
Accrued environmental assessment and remediation costs, total	2,900	2,700
Accrued environmental assessment and remediation costs, current	1,400	800
Accrued environmental assessment and remediation costs, noncurrent	1,500	1,900
Regulatory assets, total	1,600	1,700
Regulatory assets, current	1,200	1,200
Regulatory assets, noncurrent	$ 400	$ 500
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