Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Regulatory assets, current	$ 107,853	$ 97,149
Regulatory assets, noncurrent	283,986	276,315
Environmental assessment and remediation [Member]
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total	3,700	4,000
Accrued environmental assessment and remediation costs, current	1,800	1,800
Accrued environmental assessment and remediation costs, noncurrent	1,900	2,200
Regulatory assets, total	1,600	2,200
Regulatory assets, current	1,200	1,200
Regulatory assets, noncurrent	400	$ 1,000
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]
Notice of Penalty			$ 1,600