Debt (Corporate Revolving Facility and other Letters of Credit Facilities) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		$ 1,365		$ 1,069
Amendment No. 8 [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Increase (Decrease), Net	$ 220
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		693		629
Revolving Credit Facility [Member] | Amendment No. 6 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,470
Total Letter of Credit Sublimit			$ 1,150
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		251		244
Various Project Financing Facilities [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		228		196
Other Corporate Facilities [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount		193		$ 0
Line of Credit Facility, Maximum Borrowing Capacity		$ 200