Asset Retirement Obligations (Rollforward) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
Beginning asset retirement obligation	$ 106,430,000		$ 93,468,000
Liabilities incurred and assumed during period	22,216,000		50,956,000
Revisions in estimated retirement obligations	4,730,000		5,334,000
Liabilities settled and sold during period	(15,523,000)		(50,556,000)
Accretion expense	8,448,000		7,228,000
Ending asset retirement obligation	126,301,000		106,430,000
Asset retirement obligations - current	(6,413,000)	[1]	(3,700,000)	[1]
Long-term asset retirement obligation	119,888,000		102,730,000
Asset Retirement Obligations (Textuals) [Abstract]
Balances in escrow accounts	$ 36,000,000		$ 35,200,000

[1] Included in "Accounts payable and accrued liabilities" in our Consolidated Balance Sheets.