Lease Arrangements (Details 2) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Leased vehicles
Out-of-period adjustment relating to capital lease accounting
Minimum lease term required under master lease agreement				12 months
Current minimum lease term per vehicle				60 months
Decrease in pre-tax income due to out-of-period adjustment relating to capital lease accounting	$ 700,000			$ 700,000
Decrease in pre-tax income due to out-of-period adjustment relating to capital lease accounting, after tax	400,000			400,000
Increase in net property and equipment due to out-of-period adjustment relating to capital lease accounting				16,500,000
Increase in capital lease obligations due to out-of-period adjustment relating to capital lease accounting				17,300,000
Impact to the cash flow statement due to increased depreciation and payments on capital lease obligations				5,900,000
Capital Leases
2013		6,287,000
2014		3,147,000
2015		3,146,000
2016		3,127,000
2017		3,089,000
Total minimum lease payments		18,796,000
Less amounts representing interest		1,112,000
Present value of minimum lease payments		17,684,000
Less current maturities		6,052,000	89,000
Total long-term obligations under capital leases		11,632,000	342,000
Operating Leases
2013		59,837,000
2014		46,964,000
2015		38,134,000
2016		24,063,000
2017		14,401,000
Thereafter		14,456,000
Total minimum lease payments		$ 197,855,000