(Temporarily Impaired Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Temporarily Impaired Securities, Less than 12 Months, Fair Value	$ 134	$ 1,011
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(6)	(75)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	75	251
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	209	1,262
Available-for-sale Securities, Continuous Unrealized Loss Position, More than 12 Months	(15)	(11)
Available-for-sale Securities, Continuous Unrealized Position	(21)	(86)
Cost method investments, aggregate cost	189	204
Net unrealized gain recognized in equity securities	5	7
Government debt
Debt Securities, Available-for-sale [Line Items]
Temporarily Impaired Securities, Less than 12 Months, Fair Value	55	287
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)	(17)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	23	187
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	78	474
Available-for-sale Securities, Continuous Unrealized Loss Position, More than 12 Months	(8)	(6)
Available-for-sale Securities, Continuous Unrealized Position	(11)	(23)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Temporarily Impaired Securities, Less than 12 Months, Fair Value	79	724
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)	(58)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	52	64
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	131	788
Available-for-sale Securities, Continuous Unrealized Loss Position, More than 12 Months	(7)	(5)
Available-for-sale Securities, Continuous Unrealized Position	(10)	(63)
Equity securities
Debt Securities, Available-for-sale [Line Items]
Equity securities	$ 15	$ 16