Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Asset Retirement Obligations
Asset retirement obligations expected to be paid within a year	$ 23,900,000	$ 12,300,000
Asset retirement obligations expected to be incurred beyond one year	92,500,000	77,200,000
Assets legally restricted for purposes of settling asset retirement obligations	0	0
Changes in the asset retirement obligations
Balance at beginning of year	89,478,000	74,963,000
Accretion expense	9,079,000	4,411,000
Revisions in estimated cash flows	26,453,000	14,353,000
Obligation settled	(8,617,000)	(4,249,000)
Balance at end of year	$ 116,393,000	$ 89,478,000