Investments (Gains and Losses Recognized on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of losses and gains related to equity securities [Line Items]
Net realized gains (losses) on financial instruments	$ 27	$ 115	$ 5	$ 138
Equity Securities [Member]
Schedule of losses and gains related to equity securities [Line Items]
Net realized gains (losses) on financial instruments	27		(33)
Net realized gains recognized on sale of equity securities during the period	11		208
Unrealized gains (losses) recognized on equity securities still held	$ 16		$ (241)