Discontinued Operations and Assets Held for Sale (Major Categories of Assets and Liabilities Reflected as Held for Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accounts receivable	$ 243,472	$ 357,099
Total current assets held for sale	22,929	27,070
Accounts payable	595,916	784,975
Total current liabilities associated with assets held for sale	3,530	2,776
Asset retirement obligation - Seal asset	709,299	681,528
UK Refining And Marketing Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash	16,631	4,126
Accounts receivable	6,298	22,944
Total current assets held for sale	22,929	27,070
Accounts payable	837	270
Refinery decommissioning cost	2,693	2,506
Total current liabilities associated with assets held for sale	$ 3,530	2,776
Asset retirement obligation - Seal asset		$ 85,900