Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Regulatory assets, current	$ 87,996	$ 87,059
Regulatory assets, noncurrent	256,274	264,001
Environmental assessment and remediation [Member]
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total	4,000	4,200
Accrued environmental assessment and remediation costs, current	2,100	2,100
Accrued environmental assessment and remediation costs, noncurrent	1,900	2,100
Regulatory assets, total	1,900	2,500
Regulatory assets, current	1,200	1,200
Regulatory assets, noncurrent	700	1,300
Environmental assessment and remediation [Member] | Minimum [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 6,000
Expected duration of environmental assessment and remediation spending	4 years
Environmental assessment and remediation [Member] | Maximum [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 8,000
Expected duration of environmental assessment and remediation spending	6 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
Accrued Liabilities
Site Contingency [Line Items]
Potential refund obligation	$ 19,100
Notice of Penalty		$ 1,600