Recent Financing Transactions Recent Financing Transactions - Credit Facilities (Details) (Line of Credit [Member], USD $) In Millions, unless otherwise specified	1 Months Ended
	Jun. 30, 2012	Sep. 30, 2012
Unsecured credit facility, $635 million, expiring October 2012 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity replaced	$ 635
Line of Credit Facility, Expiration Date	Oct. 31, 2012
Unsecured credit facility, $600 million, expiring June 2017 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	600
Line of Credit Facility, Expiration Date	Jun. 30, 2017
Line of Credit Facility, Amount Outstanding		$ 0
Debt to capitalization ratio		0.65