Summary of Significant Accounting Policies (Details) - Updated Guidance - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2020	Dec. 31, 2018	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2017
New Accounting Pronouncements
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201601Member
Shareholders' equity		$ 29,201,000,000	$ 22,894,000,000		$ 25,943,000,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]						us-gaap:OtherAssets
Operating lease liability		$ 362,000,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]		us-gaap:OtherLiabilities				us-gaap:OtherLiabilities
Retained earnings [Member]
New Accounting Pronouncements
Shareholders' equity		$ 38,771,000,000	35,204,000,000		36,977,000,000			$ 33,462,000,000
Reclassification of certain tax effects from adoption of updated accounting guidance	$ 24,000,000
Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements
Shareholders' equity		$ 2,502,000,000	$ (1,859,000,000)		640,000,000			(343,000,000)
Reclassification of certain tax effects from adoption of updated accounting guidance	(24,000,000)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained earnings [Member]
New Accounting Pronouncements
Shareholders' equity	22,000,000			$ (43,000,000)	$ (43,000,000)			22,000,000
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements
Shareholders' equity	$ (22,000,000)							$ (22,000,000)
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements
Right-of-use asset						$ 320,000,000
Operating lease liability						$ 384,000,000
Accounting Standards Update 2016-02 [Member] | Retained earnings [Member]
New Accounting Pronouncements
Shareholders' equity							$ 0