Assets Held for Sale (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Assets Held for Sale
Assets held-for-sale reasonably certain period for sale			1
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $539,769 in 2013 and 20,676 in 2012	$ 400,393		$ 400,393		$ 33,620
Asset retirement obligation associated with oil and gas properties held for sale	25,339		25,339		1,393
Impairment of oil and gas properties	5,935	0	61,706	38,523
Oil and Gas Properties [Member]
Assets Held for Sale
Impairment of oil and gas properties	$ 8,700		$ 10,100