Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Nov. 16, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Regulatory assets, total	$ 373,500		$ 351,100
Regulatory assets, current	97,149		87,059
Regulatory assets, noncurrent	276,315		264,001
Other Commitments [Abstract]
Commitments for construction and acquisition of property, plant, and equipment	114,000
Accrued Liabilities [Member]
Site Contingency [Line Items]
Notice of Penalty			1,600
Environmental assessment and remediation
Site Contingency [Line Items]
Accrued environmental assessment and remediation costs, total	4,000		4,200
Accrued environmental assessment and remediation costs, current	1,800		2,100
Accrued environmental assessment and remediation costs, noncurrent	2,200		2,100
Regulatory assets, total	2,200		2,500
Regulatory assets, current	1,200		1,200
Regulatory assets, noncurrent	1,000		$ 1,300
Environmental assessment and remediation | Minimum
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 | Maximum
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 | Maximum
Site Contingency [Line Items]
Environmental assessment and remediation costs, best estimate	$ 500
Settled Litigation [Member]
Site Contingency [Line Items]
Rate Settlement refund		$ 19,200