Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Site Contingency [Line Items]
Regulatory assets, current	$ 85,334	$ 79,575
Regulatory assets, noncurrent	254,153	263,730
Environmental assessment and remediation [Member]
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total	2,400	2,900
Accrued environmental assessment and remediation costs, current	1,400	1,400
Accrued environmental assessment and remediation costs, noncurrent	1,000	1,500
Regulatory assets, total	700	1,600
Regulatory assets, current	700	1,200
Regulatory assets, noncurrent	0	$ 400
Environmental assessment and remediation [Member] | Minimum [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 4,000
Expected duration of environmental assessment and remediation spending	3 years
Environmental assessment and remediation [Member] | Maximum [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 6,000
Expected duration of environmental assessment and remediation spending	5 years
Potentially responsible party at various Superfund and state waste disposal sites [Member] | Maximum [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 500
Revenue
Site Contingency [Line Items]
Potential refund obligation	15,300
Interest Expense
Site Contingency [Line Items]
Potential refund obligation	3,300
Accrued Liabilities
Site Contingency [Line Items]
Potential refund obligation	18,600
Notice of Penalty	$ 1,600