Recent Financing Transactions - Credit Facilities (Details) - USD ($) $ in Millions	1 Months Ended
	Mar. 31, 2015	Jun. 30, 2015
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, Amounts Supported		$ 56
Letters of credit supporting tax-exempt bond obligations [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		$ 428
Letters of credit supporting tax-exempt bond obligations [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, Increase (Decrease), Net	$ 191
Letters of credit supported by credit facility [Member] | Letters of credit supporting tax-exempt bond obligations [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit Facility, Increase (Decrease), Net	$ (23)