Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments
Fair Value	$ 1,434	$ 1,336
Change in unrealized gains (losses) on trading securities	176	(110)	$ 146
Domestic equity securities
Investments
Fair Value	706	632
Domestic debt securities
Investments
Fair Value	409	268
Foreign equity securities
Investments
Fair Value	189	218
Foreign equity securities | Denominated in foreign currencies
Investments
Fair Value	62	66
Foreign equity securities | Denominated in Euros
Investments
Fair Value	32	25
Foreign equity securities | Denominated in Japanese Yen
Investments
Fair Value	12	20
Foreign equity securities | Denominated in British pounds
Investments
Fair Value	8	9
Foreign equity securities | Denominated in other foreign currencies
Investments
Fair Value	10	12
High yield securities
Investments
Fair Value	56	19
Foreign debt securities
Investments
Fair Value	43	16
Foreign debt securities | Denominated in Euros
Investments
Fair Value	13
Collateralized loan obligation
Investments
Fair Value	15	28
Money market funds held in trading accounts
Investments
Fair Value	12	146
Other trading securities
Investments
Fair Value	4	9
Money market funds | Denominated in other foreign currencies
Investments
Fair Value		$ 10
Money market funds | Denominated in other foreign currencies | Maximum
Investments
Fair Value	$ 1