Goodwill and Other Intangibles Puchase Price Allocation (Details) (Credo [Member], USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Credo [Member]
Business Acquisition [Line Items]
Cash and short-term investments	$ 2,300		$ 2,300
Measurememt period adjustment, cash and short-term investments	0
Receivables	10,147		9,144
Measurememt period adjustment, receivables	1,003	[1]
Oil and gas properties and equipment	135,802		140,514
Measurememt period adjustment, oil and gas properties and equipment	(4,712)	[2]
Other properties and equipment	67		67
Measurememt period adjustment, goodwill and other intangible assets	0
Goodwill and other intangible assets	61,209		58,396
Measurememt period adjustment, other	2,813	[3]
Other	676		676
Measurememt period adjustment, total assets acquired	0
Total assets acquired	210,201		211,097
Measurememt period adjustment, accounts payable and accrued liabilities	(896)
Accounts payable and accrued liabilities	29,940		29,927
Measurememt period adjustment, deferred tax liability	13	[4]
Deferred tax liability	23,791		24,700
Measurememt period adjustment, other liabilities	(909)	[5]
Other liabilities	1,255		1,255
Measurememt period adjustment, total liabilities assumed	0
Total liabilities assumed	54,986		55,882
Measurememt period adjustment, estimated fair value of net assets acquired	(896)
Estimated fair value of net assets acquired	155,215		155,215
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred	$ 0

[1] Primarily related to unrecorded seismic and leasehold costs due from partners.

[2] Fair value adjustments allocated to near-term expiring leasehold acreage.

[3] Goodwill adjustments associated with fair value adjustments for oil and gas properties, net of deferred taxes and working capital adjustments.

[4] Primarily related to current income taxes payable.

[5] Primarily related to deferred taxes on fair value adjustments of near-term expiring leasehold acreage.