Pension and Other Post-Employment Benefits (Plan Assets By Investment Asset Category And Fair Value Input Level) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	$ 210	$ 194
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	28	28
Fixed Income - Canadian Bondfunds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	67	61
Domestic Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	54	50
Foreign Equity[Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	50	45
Real Estate And Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	11	10
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	40	39
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	27	27
Level 1 [Member] | Domestic Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	13	12
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	159	145
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	1	1
Level 2 [Member] | Fixed Income - Canadian Bondfunds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	67	61
Level 2 [Member] | Domestic Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	41	38
Level 2 [Member] | Foreign Equity[Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	50	45
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	11	10
Level 3 [Member] | Real Estate And Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets, End of Year	$ 11	$ 10	$ 10