Notes Payable (Textuals) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 08, 2011
Notes Payable [Abstract]
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				600,000,000
Borrowing rate under revolving loan facility				137.50%
Contracutal interest rate on borrowing under facility	1.59%
Commitment fees basis points for revolving loan facility	20.00%
Remaining borrowing capacity under Revolving Loan Facility as reduced by outstanding letters of credit	305,100,000
Leverage ratio on restrictive financial covenants	35.00%
Letter of credit facility outstanding amount	90,000,000
Line of credit facility expiration date	Dec. 31, 2016
Repayment of convertible debt	0	0	64,472,000
Subsidiary letter of credit facility outstanding amount	10,400,000
Senior Notes Payable Fair Value	$ 351,400,000	$ 325,700,000