Significant Accounting Policies (Adoption of ASUs, Changes To Statement Of Stockholder's Equity) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 2,741	$ 2,322	$ 2,204	$ 1,167
Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity		3	16
Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	(39)	9	5	5
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				7
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity			2
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2016-01 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				(2)
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				4
Retained Earnings (Accumulated Deficit) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 841	291	156	(895)
Retained Earnings (Accumulated Deficit) [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				(881)
Retained Earnings (Accumulated Deficit) [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity		$ 3	$ 14
Retained Earnings (Accumulated Deficit) [Member] | Impact of adopting ASC 606 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				16
Retained Earnings (Accumulated Deficit) [Member] | Accounting Standards Update 2016-01 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				2
Retained Earnings (Accumulated Deficit) [Member] | Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				$ (4)