Debt Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Debt Obligations [Abstract]
Long-term notes payable	$ 350,000,000	$ 0
Less current portion of long-term debt	(17,500,000)	0
Long-term debt	332,500,000	0
Scheduled maturities of long-term debt [Abstract]
2013	26,250,000
2014	52,500,000
2015	87,500,000
2016	166,250,000
Total	332,500,000	0
Line of credit facility [Abstract]
Term of credit facility (in years)	5Y
Advance from unsecured resolving credit facility	175,000,000
Maximum borrowing capacity under term loan	350,000,000
Maturity of Bank Facility	November 30, 2016
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 such Interest Period multiplied by (ii) the Statutory Reserve Rate, plus (iii) the Applicable Margin.
Line of Credit Facility, Unused Capacity, Minimum Commitment Fee Percentage	0.25%
Line of Credit Facility, Unused Capacity, Maximum Commitment Fee Percentage	0.50%
Outstanding term loan	350,000,000
Outstanding letters of credit	13,800,000
Available borrowing capacity under unsecured revolving line of credit	161,200,000
Interest rate (in hundredths)	3.06%
Interest expense paid	$ 900,000