Notes Payable (Textuals) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011 rate	Dec. 31, 2010	Dec. 31, 2009	Dec. 19, 2011	Mar. 08, 2011 rate
Notes Payable [Abstract]
Notes payable at fair value		$ 325,700,000	$ 314,000,000
Percentage of senior notes		6.30%
Senior note, face amount				300,000,000
Senior note, unamortized discount		1,500,000
Senior note, effective interest rate		6.37%
Borrowing capacity under line of credit facility					575,000,000	600,000,000
Borrowing rate under revolving loan facility						137.5
Contracutal interest rate on borrowing under facility		1.65%
Commitment fees basis points for revolving loan facility		0.20%
Remaining borrowing capacity under Revolving Loan Facility as reduced by outstanding letters of credit		407,400,000
Leverage ratio on restrictive financial covenants		35.00%
Letter of credit facility outstanding amount		90,000,000
Line of credit facility expiration date		December 31, 2015
Outstanding letters of credit reduced our borrowing capacity		12,600,000
Percentage of convertible debt				1.30%
Repayment of convertible debt	64,500,000	0	64,472,000	60,210,000
Subsidiary letter of credit facility outstanding amount		$ 13,100,000