Debt (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Debt [Abstract]
Long-term notes payable	$ 335,454,000		$ 335,454,000		$ 350,000,000
Less current portion of long-term debt	(21,875,000)		(21,875,000)		(17,500,000)
Long-term debt	313,579,000		313,579,000		332,500,000
Scheduled maturities of long-term debt [Abstract]
2013	21,875,000		21,875,000
2014	52,500,000		52,500,000
2015	87,500,000		87,500,000
2016	151,704,000		151,704,000
Total	313,579,000		313,579,000		332,500,000
Line of credit facility [Abstract]
Term of credit facility	5 years		5 years
Advance from unsecured resolving credit facility			175,000,000
Maximum borrowing capacity under term loan	350,000,000		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 such 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	310,500,000		310,500,000
Outstanding letters of credit	11,500,000		11,500,000
Interest rate (in hundredths)	3.00%		3.00%
Interest expense paid	2,800,000		5,700,000
Available borrowing capacity under unsecured revolving line of credit	138,500,000		138,500,000
Issuance Of Equity Disclosure [Abstract]
Stock issued in initial public offering (in shares)		2,875,000
Proceeds from equity offering		217,600,000	217,565,000	0
Proceeds from issuance initial public offering used for repayment of debt		$ 30,800,000