Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Debt [Abstract]
Long-term notes payable	$ 320,139,000	$ 326,704,000
Less current portion of long-term debt	(32,813,000)	(26,250,000)
Long-term debt	287,326,000	300,454,000
Scheduled maturities of long-term debt [Abstract]
2013	19,688,000
2014	52,500,000
2015	87,500,000
2016	160,451,000
Long-term notes payable	320,139,000	326,704,000
Line of Credit Facility [Line Items]
Term of credit facility	5 years
Advance from revolving credit facility	175,000,000
Maximum borrowing capacity under term loan	350,000,000
Maturity of bank facility	2016-11-30
Line of credit facility, interest rate description	Borrowings under the Bank Facility bear interest, at the Company's option, at either (A) the highest of (i) the Prime Rate, (ii) the Federal Funds Effective Rate plus 0.5%, and (iii) the Adjusted LIBO Rate for an Interest Period of one month plus 1.0%, in each case plus an Applicable Margin based on the Company's Leverage Ratio or (B) the interest rate equal to (i) the rate for US dollar deposits in the London interbank market for the applicable Interest Period multiplied by (ii) the Statutory Reserve Rate, plus (iii) the Applicable Margin.
Minimum unused commitment fee (in hundredths)	0.25%
Maximum unused commitment fee (in hundredths)	0.50%
Outstanding term loan	295,100,000
Outstanding line of credit	25,000,000
Funded debt letters of credit	11,200,000
Interest rate (in hundredths)	2.75%
Interest expense paid	2,100,000
Available borrowing capacity under unsecured revolving line of credit	$ 138,800,000
Federal Funds Effective Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (in hundredths)	0.50%
LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (in hundredths)	1.00%