Assets and Liabilities Measured at Fair Value (Nonrecurring Fair Value Measurements) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015	[1]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net Book Value Prior to Impairment		$ 167,055,000
Total Pretax (Noncash) Impairment Expense	$ 0	95,088,000	[1]	$ 2,493,156,000
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets: Impaired proved properties		71,967,000
Western Canada [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net Book Value Prior to Impairment		167,055,000
Total Pretax (Noncash) Impairment Expense		95,088,000
Western Canada [Member] | Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets: Impaired proved properties		$ 71,967,000

[1] Reclassified to conform to current presentation (see Note A).