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Significant Accounting Policies (Narrative) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Significant Accounting Policies [Line Items] |  |  |  |  |
Maturity of Canadian government securities | 90 days | 90 days |  | 90 days |
Canadian government securities with maturities greater than 90 days at the date of acquisition |  | $ 111,542,000 |  |  |
Impairment of assets | $ 0 | 95,088,000 | [1] | $ 2,493,156,000 | [1]
Withholding tax on amounts repatriated | 5.00% |  |  |  |
Stock Options [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Stock-based compensation, fair value assumption model | Black-Scholes option pricing model |  |  |  |
Performance Based Restricted Stock [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Stock-based compensation, fair value assumption model | Monte Carlo valuation model |  |  |  |
Stock-based compensation, vesting period | 3 years |  |  |  |
Minimum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Equity method investment, ownership percentage | 20.00% |  |  |  |
Minimum [Member] | Stock Options [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Stock-based compensation, vesting period | 2 years |  |  |  |
Maximum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Equity method investment, ownership percentage | 50.00% |  |  |  |
Maximum [Member] | Stock Options [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Stock-based compensation, vesting period | 3 years |  |  |  |
Syncrude [Member] | Minimum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Turnarounds for major processing units | 2 years |  |  |  |
Syncrude [Member] | Maximum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Turnarounds for major processing units | 3 years |  |  |  |
Natural Gas Processing Facilities [Member] | Minimum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Depreciable lives, straight-line method | 20 years |  |  |  |
Natural Gas Processing Facilities [Member] | Maximum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Depreciable lives, straight-line method | 25 years |  |  |  |
Canada [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Impairment of assets |  | $ 95,088,000 |  | $ 683,574,000 |
[1] | Reclassified to conform to current presentation (see Note A).

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