Business, Basis of Presentation and Significant Accounting Policies - Adoption of Accounting Pronouncements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) segments	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2016 USD ($)
New Accounting Pronouncement, Early Adoption [Line Items]
Impairment write-down resulting in a premium deficiency reserve			$ 43
Other assets	$ 902	$ 1,027
Operating expenses	2,746	2,606	2,562
Other assets	$ 10,218	11,292
Number of operating segments | segments	3
Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			0
Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU					$ 15
Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			84
Accounting Standards Update 2018-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU	$ 0
Accounting Standards Update 2016-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU	146	$ 0	0
Other liabilities | Accounting Standards Update 2016-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU				$ 146
Other assets | Accounting Standards Update 2016-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU				146
Unappropriated | Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			28
Unappropriated | Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU					15
Unappropriated | Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			84
Unappropriated | Accounting Standards Update 2018-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU	(343)			343
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			(28)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU					$ 0
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU			$ 0
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02
New Accounting Pronouncement, Early Adoption [Line Items]
Adjustment for adoption of ASU	$ 343			$ 343