Long-term Debt and Capital Lease Obligations (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt, total	$ 292,870	$ 260,699
Less current installments	6,561	5,632
Total long-term debt and capital lease obligations	286,309	255,067
Term Loan Under Credit Agreement, Variable [Member]
Debt Instrument [Line Items]
Long-term debt, total	223,875	225,000
Revolver Under Credit Agreement, Variable [Member]
Debt Instrument [Line Items]
Long-term debt, total	34,236	6,236
Capital Leases [Member]
Debt Instrument [Line Items]
Long-term debt, total	15,181	15,316
Fixed interest rate (in hundredths)	3.85%
Debt, face amount	411
Capital Leases [Member] | Minimum [Member]
Debt Instrument [Line Items]
Fixed interest rate, minimum (in hundredths)	2.04%	3.00%
Capital Leases [Member] | Maximum [Member]
Debt Instrument [Line Items]
Fixed interest rate, maximum (in hundredths)	7.73%	7.73%
Notes Payable [Member]
Debt Instrument [Line Items]
Long-term debt, total	11,615	6,034
Notes Payable [Member] | Maximum [Member]
Debt Instrument [Line Items]
Fixed interest rate, maximum (in hundredths)	3.60%	3.25%
Missouri IRBs [Member]
Debt Instrument [Line Items]
Long-term debt, total	7,963	8,113
Fixed interest rate (in hundredths)	2.80%	2.80%
Promissory Note to Finance Purchase of Corporate Aircraft [Member]
Debt Instrument [Line Items]
Fixed interest rate (in hundredths)	3.60%
Debt, face amount	3,550
Promissory Note to Finance Building [Member]
Debt Instrument [Line Items]
Fixed interest rate (in hundredths)	2.95%
Debt, face amount	$ 2,200