Short And Long-Term Debt Other Debt Facilities (Details) - USD ($) $ in Millions	1 Months Ended
	Jun. 30, 2016	Sep. 30, 2016	Mar. 31, 2016	Oct. 23, 2015
Line of Credit Facility [Line Items]
Letter of Credit, borrowing capacity		$ 125		$ 125
Unsecured Letter of Credit Facility
Line of Credit Facility [Line Items]
Increase in borrowing capacity	$ 47
Letter of Credit, borrowing capacity		135	$ 88
Outstanding letters of credit		128
Unsecured Letter of Credit Facility \| June 2017
Line of Credit Facility [Line Items]
Letter of Credit, borrowing capacity		53
Unsecured Letter of Credit Facility \| September 2018
Line of Credit Facility [Line Items]
Letter of Credit, borrowing capacity		16
Unsecured Letter of Credit Facility \| December 2019
Line of Credit Facility [Line Items]
Letter of Credit, borrowing capacity		$ 66