Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Jul. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Trade and other		$ 305,949	$ 251,872
Regulatory assets, current		90,367	87,059
Regulatory assets, noncurrent		269,000	264,001
Duration Of Period For Deferred Revenue Recognition	25 years
Environmental assessment and remediation [Member]
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total		3,900	4,200
Accrued environmental assessment and remediation costs, current		2,100	2,100
Accrued environmental assessment and remediation costs, noncurrent		1,800	2,100
Regulatory assets, total		1,600	2,500
Regulatory assets, current		1,200	1,200
Regulatory assets, noncurrent		400	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
Potential refund obligation
Site Contingency [Line Items]
Trade and other		$ 19,300
Accrued Liabilities
Site Contingency [Line Items]
Notice of Penalty			$ 1,600