Investments, Investment Income and Gains and Losses - Short-Term Investments and Cash Equivalents (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short Term Investments And Cash Equivalents [Line Items]
Net proceeds on issuance of long-term debt	$ 0	$ 22,856	$ 1,581
Corporate Securities [Member]
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents		581
Short-term investments and cash equivalents, including disposal groups	1,985
Federal goverment securities
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents		82
Short-term investments and cash equivalents, including disposal groups	472
Foreign government [Member]
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents		238
Short-term investments and cash equivalents, including disposal groups	65
Money Market Funds [Member]
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents		$ 1,174
Short-term investments and cash equivalents, including disposal groups	$ 631