Long-Term Debt (Line of Credit Amendments) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Mandatory Redemption Date 2	Dec. 31, 2013
Long-term Debt	$ 1,200,472,000	$ 1,036,305,000
Line Of Credit Facility Base Interest Rate	5.00%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.75%
Line of Credit Facility, Expiration Date	Mar. 05, 2016
Line Of Credit Facility Collateral Units	10,700,000
Line Of Credit Amendment Date2
Line of Credit Facility [Line Items]
Line of credit facility, amended date	8/28/2013
Line Of Credit Amendment Date1
Line of Credit Facility [Line Items]
Line of credit facility, amended date	1/28/2013
Line Of Credit Amendment Date 3 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, amended date	1/22/2014
Eurodollar Rate [Member]
Leverage Ratios [Abstract]
Percentage Rate	1.00%
Eurodollar Rate [Member] | Greater than or equal to 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	3.00%
Eurodollar Rate [Member] | Greater than or equal to 4.00 to 1.00 and less than 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	2.75%
Eurodollar Rate [Member] | Greater than or equal to 3.50 to 1.00 and less than 4.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	2.50%
Eurodollar Rate [Member] | Greater than or equal to 3.00 to 1.00 and less than 3.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	2.25%
Eurodollar Rate [Member] | Less than 3.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	2.00%
Letter of Commitment Fee [Member] | Greater than or equal to 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	0.50%
Letter of Commitment Fee [Member] | Greater than or equal to 4.00 to 1.00 and less than 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	0.50%
Letter of Commitment Fee [Member] | Greater than or equal to 3.50 to 1.00 and less than 4.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	0.50%
Letter of Commitment Fee [Member] | Greater than or equal to 3.00 to 1.00 and less than 3.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	0.50%
Letter of Commitment Fee [Member] | Less than 3.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	0.38%
Base Rate [Member]
Leverage Ratios [Abstract]
Percentage Rate	0.50%
Base Rate [Member] | Greater than or equal to 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	2.00%
Base Rate [Member] | Greater than or equal to 4.00 to 1.00 and less than 4.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	1.75%
Base Rate [Member] | Greater than or equal to 3.50 to 1.00 and less than 4.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	1.50%
Base Rate [Member] | Greater than or equal to 3.00 to 1.00 and less than 3.50 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	1.25%
Base Rate [Member] | Less than 3.00 to 1.00
Leverage Ratios [Abstract]
Percentage Rate	1.00%
Credit Facility
Line of Credit Facility [Line Items]
Postamendment borrowing capacity	635,000,000
Borrowed under existing credit facility	59,700,000.0
Avaliable borrowing capacity	420,300,000
Leverage Ratios [Abstract]
Available Additional Borrowings	207,100,000
Minimum [Member] | March 31, 2014, June 30, 2014, September 30, 2014 and December 31, 2014 [Member]
Leverage Ratios [Abstract]
Interest Coverge Ratio	2.25 to 1.0
Minimum [Member] | After December 31, 2014 [Member]
Leverage Ratios [Abstract]
Interest Coverge Ratio	2.50 to 1.0
Minimum [Member] | Credit Facility
Leverage Ratios [Abstract]
Line of Credit Facility, Commitment Fee Percentage	0.375%
Maximum [Member]
Leverage Ratios [Abstract]
Senior Leverage Ratio	2.75 to 1.0
Maximum [Member] | March 31, 2014, June 30, 2014 and September 30, 2014 [Member]
Leverage Ratios [Abstract]
Interest Coverage Ratio	5.50 to 1.0
Maximum [Member] | December 31, 2014 And Thereafter [Member]
Leverage Ratios [Abstract]
Interest Coverage Ratio	5.25 to 1.0
Maximum [Member] | Credit Facility
Leverage Ratios [Abstract]
Line of Credit Facility, Commitment Fee Percentage	0.50%
CEI Credit Facility [Member]
Line of Credit Facility [Line Items]
Long-term Debt	65,041,000
Avaliable borrowing capacity	25,000,000
Pre Amendment [Member] | CEI Credit Facility [Member]
Line of Credit Facility [Line Items]
Postamendment borrowing capacity	75,000,000.0
Post Amendment [Member] | CEI Credit Facility [Member]
Line of Credit Facility [Line Items]
Postamendment borrowing capacity	90,000,000.0
2016 | CEI Credit Facility [Member]
Line of Credit Facility [Line Items]
Long-term Debt		$ 0
Subsidiary Borrower [Domain]
Leverage Ratios [Abstract]
Line of Credit Facility, Initiation Date	Mar. 05, 2013