Debt Debt, Textuals (Details) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2013 February 2013 [Member]	Dec. 31, 2011 February 2013 [Member]	Dec. 31, 2012 October 2014 [Member]	Dec. 31, 2011 April 2015 [Member]	Dec. 31, 2012 April 2015 [Member]	Dec. 31, 2011 April 2016 [Member]	Dec. 31, 2012 April 2016 [Member]	Dec. 31, 2012 June 2017 [Member]	Dec. 31, 2012 May 2019 [Member]	Dec. 31, 2012 November 2021 [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.28%	6.69%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	87.00%	89.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000		$ 400,000,000
Letters of Credit, Maximum Borrowing Capacity			175,000,000
Line of Credit Facility, Amount Outstanding	86,600,000	65,000,000
Letters of Credit Outstanding, Amount	33,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Line of Credit Facility, Remaining Borrowing Capacity	380,400,000
Proceeds from Issuance of Senior Long-term Debt	200,000,000
Debt Instrument, Interest Rate, Stated Percentage					7.875%	7.375%	6.75%	6.75%	6.75%	6.75%	7.00%	4.50%	8.00%
Repayments of Long-term Capital Lease Obligations	25,760,000	23,030,000	46,214,000
Repayments of Long-term Debt	206,600,000	46,000,000		4,800,000	3,800,000	180,700,000	20,800,000		15,600,000
Losses on early extinguishment of debt, net	(22,706,000)	(3,509,000)	(9,400,000)
Write off of Deferred Debt Issuance Cost	1,300,000	400,000
Debt Instrument, Unamortized Premium	21,400,000	3,100,000
Capital Lease Obligations Incurred	78,900,000	31,300,000	22,800,000
Pledged Assets Separately Reported, Finance Receivables Pledged as Collateral, at Fair Value	1,700,000	4,100,000
Debt Instrument, Unamortized Discount	4,300,000	4,900,000
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	$ 3,100,000	$ 2,800,000