Business, Basis of Presentation and Significant Accounting Policies - Adoption of Accounting Pronouncements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) segments	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2015 USD ($)
New Accounting Pronouncement, Early Adoption [Line Items]
Impairment write-down resulting in a premium deficiency reserve	$ 43
Other assets	1,336	$ 1,310
Operating expenses	2,691	2,654	$ 2,655
Other assets	$ 16,040	15,394
Number of operating segments | segments	4
Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		0
Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			15
Accounting Standards Update 2014-13
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ (18)
Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		84
Scenario, Forecast | Accounting Standards Update 2018-02
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 343
Scenario, Forecast | Accounting Standards Update 2016-02
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 150
Unappropriated | Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		28
Unappropriated | Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			15
Unappropriated | Accounting Standards Update 2014-13
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Unappropriated | Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		84
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		(28)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			$ 0
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2014-13
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 0
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 0