Notes Payable (Textuals) (USD $)	Sep. 30, 2012
Notes Payable [Abstract]
Percentage of senior notes	6.30%
Senior note, effective interest rate	6.30%
Borrowing capacity under line of credit facility	$ 600,000,000
Contracutal interest rate on borrowing under facility	1.60%
Remaining borrowing capacity under Revolving Loan Facility as reduced by outstanding letters of credit	340,100,000
Letter of credit facility outstanding amount	$ 90,000,000