Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Site Contingency [Line Items]
Regulatory assets	$ 37,504	$ 42,362	$ 49,004
Long-term regulatory assets	348,284	357,530	344,969
Environmental Restoration Costs
Site Contingency [Line Items]
Deferred costs and interest	50,131	53,039	53,065
Accrued site liabilities	120,485	119,443	120,075
Insurance proceeds and interest	(99,884)	(98,523)	(97,547)
Total regulatory asset deferral	70,732	73,959	75,593
Regulatory assets	6,724	9,989	9,610
Long-term regulatory assets	$ 64,008	$ 63,970	$ 65,983