Debt (Debt Textuals) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Apr. 11, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instruments [Abstract]
Debt Instrument, Interest Rate, Effective Percentage			5.60%		5.40%
Subsequent Event [Member] | Short Term Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 300
Number Of Days In Which Unsecured Borrowings Must Be Converted To Secured Borrowings	21 days
Debt Instrument Term In Days	364 days
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,790
Revolving Credit Facility [Member] | Amendment No. 6 [Member]
Line of Credit Facility [Line Items]
Future Line of Credit Facility Maximum Borrowing Capacity				1,300
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,470
Total Letter of Credit Sublimit			$ 1,150
Revolving Credit Facility [Member] | Amendment No. 7 [Member]
Line of Credit Facility [Line Items]
Total Letter of Credit Sublimit			1,300
Incremental Revolving Facilities			$ 500