Recently Issued Accounting Pronouncements (Textuals) (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income ($2 million or less for each quarter)	$ 113				$ 86
Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income ($2 million or less for each quarter)		$ 2	$ 2	$ 2	$ 2	$ 2	$ 2	$ 2	$ 2	$ 4
AOCI Attributable to Parent
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption		(17)								(17)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption		$ 17								$ 17