Significant Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
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	$ 173,288,000	[1]
Impairment of assets		$ 95,088,000	2,493,156,000		$ 51,314,000
Goodwill	$ 0				0
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
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
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
Canada [Member]
Significant Accounting Policies [Line Items]
Impairment of assets	$ 37,047,000	$ 95,088,000	$ 683,574,000		$ 37,047,000	[2]

[1] Reclassified to conform to current presentation. See Note B for additional information.

[2] This amount represented the writeoff of goodwill associated with an oil and gas company acquired in 2000.