Debt Debt, Textuals (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2011 February 2013 [Member]	Mar. 31, 2011 February 2013 [Member]	Jun. 30, 2011 April 2015 [Member]	Mar. 31, 2011 April 2015 [Member]	Jun. 30, 2011 April 2016 [Member]	Mar. 31, 2011 April 2016 [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.59%		6.59%		6.69%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	88.00%		88.00%		89.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000		$ 500,000,000
Letters of Credit, Maximum Borrowing Capacity					175,000,000
Line of Credit Facility, Amount Outstanding	72,000,000		72,000,000		65,000,000
Letters of Credit Outstanding, Amount	32,400,000		32,400,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.35%
Line of Credit Facility, Remaining Borrowing Capacity	395,600,000		395,600,000
Repayments of Long-term Capital Lease Obligations			12,823,000	11,166,000
Repayments of Debt				28,100,000		3,700,000		15,600,000		7,000,000
Debt Instrument, Interest Rate, Stated Percentage							7.875%		6.75%		6.75%
Gains (Losses) on Extinguishment of Debt	0	(1,835,000)	0	(2,149,000)
Capital Lease Obligations Incurred			$ 51,000,000	$ 19,100,000