Long-term Debt - U.S. Bank Debt (Details) (USD $)	6 Months Ended					3 Months Ended	6 Months Ended	3 Months Ended					6 Months Ended			3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 U.S. bank debt and receivables facility [Member]	Dec. 31, 2011 U.S. bank debt and receivables facility [Member]	Jun. 30, 2011 Revolving credit and term loan facilities [Member]	Jun. 30, 2012 Term loan facility [Member]	Jun. 30, 2011 Revolving credit facility [Member]	Jun. 30, 2012 Revolving credit facility [Member]	Dec. 31, 2011 Revolving credit facility [Member]	Jun. 30, 2012 Letters of credit [Member]	Dec. 31, 2011 Letters of credit [Member]	Jun. 30, 2012 Parent [Member]	Jun. 30, 2011 Parent [Member]	Dec. 31, 2011 Parent [Member]	Jun. 30, 2011 Refinancing of Debt [Member] Revolving credit and term loan facilities [Member]	Jun. 30, 2012 Fair Value, Inputs, Level 2 [Member] Term loan facility [Member]	Dec. 31, 2011 Fair Value, Inputs, Level 2 [Member] Term loan facility [Member]
Debt Instrument [Line Items]
Revolving Credit Facility, Maximum Borrowing Capacity									$ 125,000,000
Interest rate, LIBOR plus							3.00%		3.25%
Debt Instrument, Face Amount							225,000,000
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum							1.25%
Term Loan Facility, Required Prepay Portion Under Excess Cash Flow Sweep Provision [Line Items]							5,000,000
Letters of Credit, Maximum Borrowing Capacity											50,000,000
Letters of Credit Outstanding, Amount											23,300,000	23,900,000
Revolving Credit Facility, Amount Outstanding									0	0
Revolving Credit Facility, Remaining Borrowing Capacity									101,700,000	101,100,000
Debt Instrument, Unused Borrowing Capacity, Amount				191,700,000	158,800,000
Investments in subsidiaries	0		0										484,290,000		412,840,000
Payments of Financing Costs	4,880,000	6,570,000				6,600,000							4,880,000	0
Deferred Finance Costs, Noncurrent, Net																4,200,000
Debt Issuance Cost																2,400,000
Write off of Deferred Debt Issuance Cost								$ 1,600,000
Long-term Debt, Fair Value, % of par value																	99.30%	99.00%