Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Site Contingency [Line Items]
Regulatory assets	$ 45,900	$ 45,781	$ 34,874
Long-term regulatory assets	343,037	356,608	349,057
Environmental Restoration Costs
Site Contingency [Line Items]
Deferred costs and interest	44,686	45,546	49,373
Accrued site liabilities	122,962	126,950	119,623
Insurance proceeds and interest	(95,100)	(94,170)	(99,195)
Regulatory Assets	72,548	78,326	69,801
Regulatory assets	5,818	6,198	7,574
Long-term regulatory assets	$ 66,730	$ 72,128	$ 62,227