Recent Financing Transactions Recent Financing Transactions - Credit Facilities (Details) (USD $) In Millions, unless otherwise specified				1 Months Ended
	Jun. 30, 2014 Letter of Credit [Member]	Jun. 30, 2014 Unenhanced Variable-Rate Tax-Exempt Bond Obligations [Member]	Jun. 30, 2014 Letters of credit supporting tax-exempt bond obligations [Member]	Mar. 31, 2014 Letters of credit supporting tax-exempt bond obligations [Member] Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, Amount Canceled				$ 97
Letters of Credit Outstanding, Amount			451
Line of Credit Facility, Amounts Supported	$ 270	$ 142