Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Site Contingency [Line Items]
Regulatory assets	$ 41,092	$ 45,781	$ 37,504
Long-term regulatory assets	339,177	356,608	348,284
Annual tariff rider collection	5,000
Annual insurance proceeds to apply against remediation costs	5,000
Total annual remediation expense and interest	10,000
Environmental Restoration Costs
Site Contingency [Line Items]
Deferred costs and interest	46,862	45,546	50,131
Accrued site liabilities	119,712	126,950	120,485
Insurance proceeds and interest	(95,824)	(94,170)	(99,884)
Regulatory Assets	70,750	78,326	70,732
Regulatory assets	5,594	6,198	6,724
Long-term regulatory assets	$ 65,156	$ 72,128	$ 64,008