Debt Debt, Textuals (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2011 February 2013 [Member]	Mar. 31, 2011 February 2013 [Member]	Sep. 30, 2011 April 2015 [Member]	Mar. 31, 2011 April 2015 [Member]	Sep. 30, 2011 April 2016 [Member]	Mar. 31, 2011 April 2016 [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.54%		6.54%		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	77,000,000		77,000,000		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	390,600,000		390,600,000
Repayments of Long-term Capital Lease Obligations			19,303,000	17,186,000
Repayments of Debt				43,100,000		3,900,000		22,300,000		16,900,000
Debt Instrument, Interest Rate, Stated Percentage							7.875%		6.75%		6.75%
Gains (Losses) on Extinguishment of Debt	0	(1,355,000)	0	(3,504,000)
Capital Lease Obligations Incurred			$ 67,000,000	$ 27,100,000