Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Site Contingency [Line Items]
Regulatory assets	$ 49,504	$ 42,362	$ 55,280
Long-term regulatory assets	345,352	357,530	341,188
Environmental Restoration Costs
Site Contingency [Line Items]
Deferred costs and interest	52,888	53,039	54,704
Accrued site liabilities	117,388	119,443	117,202
Insurance proceeds and interest	(100,575)	(98,523)	(97,893)
Total regulatory asset deferral	69,701	73,959	74,013
Regulatory assets	6,362	9,989	9,734
Long-term regulatory assets	$ 63,339	$ 63,970	$ 64,279