Contingent Liabilities and Commitments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Site Contingency [Line Items]
Regulatory assets, current	$ 86,029	$ 79,575
Regulatory assets, noncurrent	261,142	263,730
Environmental assessment and remediation [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, low estimate	4,000
Environmental assessment and remediation costs, high estimate	6,000
Accrued environmental assessment and remediation costs, total	2,700	2,900
Accrued environmental assessment and remediation costs, current	1,400	1,400
Accrued environmental assessment and remediation costs, noncurrent	1,300	1,500
Regulatory assets, total	1,300	1,600
Regulatory assets, current	1,200	1,200
Regulatory assets, noncurrent	$ 100	$ 400
Environmental assessment and remediation [Member] | Minimum [Member]
Site Contingency [Line Items]
Expected duration of environmental assessment and remediation spending	3 years
Environmental assessment and remediation [Member] | Maximum [Member]
Site Contingency [Line Items]
Expected duration of environmental assessment and remediation spending	5 years
Potentially responsible party at various Superfund and state waste disposal sites [Member]
Site Contingency [Line Items]
Environmental assessment and remediation costs, high estimate	$ 500
Revenue
Site Contingency [Line Items]
Potential refund obligation	15,200
Interest Expense
Site Contingency [Line Items]
Potential refund obligation	3,000
Accrued Liabilities
Site Contingency [Line Items]
Potential refund obligation	$ 18,200