Quarterly Operating Results (Unaudited) - Realized gains (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities									$ 289	$ 287	$ (266)
Other Security Investments
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities	$ 384	$ 15	$ 2	$ (16)	$ (2)	$ (3)	$ 12	$ 2	385	9
Equity securities
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities									(96)	277	$ (265)
Equity securities | Accounting Standards Update 2016-01
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities	$ 207	$ 30	$ 202	$ (535)	$ 67	$ (15)	$ 44	$ 182	$ (96)	$ 278