Shareholders' Equity, Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	$ 19,181	$ 21,930
Other comprehensive loss	(611)	(1,144)
Balance	18,537	19,178
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	581	(68)
Other comprehensive income (loss) before reclassifications, after tax	(590)	(1,216)
Reclassification of losses from accumulated other comprehensive income, after tax	(21)	72
Other comprehensive loss	(611)	(1,144)
Amounts attributable to noncontrolling interests	64	119
Balance	34	(1,093)
Net Unrealized Gains (Losses) on Investments with an Allowance for Credit Losses [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	0	0
Other comprehensive income (loss) before reclassifications, after tax	(3)	(48)
Reclassification of losses from accumulated other comprehensive income, after tax	3	37
Other comprehensive loss	0	(11)
Amounts attributable to noncontrolling interests		1
Balance	0	(10)
Other Comprehensive Income (Loss), Tax [Abstract]
Other comprehensive income (loss) before reclassifications, tax	1	13
Reclassification of (income) losses from accumulated other comprehensive income, tax	(1)	(10)
Net Unrealized Gains (Losses) on Other Investments [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	1,563	918
Other comprehensive income (loss) before reclassifications, after tax	(593)	(1,066)
Reclassification of losses from accumulated other comprehensive income, after tax	(34)	22
Other comprehensive loss	(627)	(1,044)
Amounts attributable to noncontrolling interests	66	111
Balance	1,002	(15)
Other Comprehensive Income (Loss), Tax [Abstract]
Other comprehensive income (loss) before reclassifications, tax	154	287
Reclassification of (income) losses from accumulated other comprehensive income, tax	8	(6)
Unrealized Gains (Losses) on Cash Flow Hedges [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	(23)	(6)
Other comprehensive income (loss) before reclassifications, after tax	3	(19)
Reclassification of losses from accumulated other comprehensive income, after tax	1
Other comprehensive loss	4	(19)
Balance	(19)	(25)
Other Comprehensive Income (Loss), Tax [Abstract]
Other comprehensive income (loss) before reclassifications, tax	(2)	6
Reclassification of (income) losses from accumulated other comprehensive income, tax	(1)	0
Pension and Postretirement Benefits [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	(877)	(855)
Other comprehensive income (loss) before reclassifications, after tax		1
Reclassification of losses from accumulated other comprehensive income, after tax	9	13
Other comprehensive loss	9	14
Amounts attributable to noncontrolling interests	(1)	(1)
Balance	(869)	(842)
Other Comprehensive Income (Loss), Tax [Abstract]
Other comprehensive income (loss) before reclassifications, tax	0	0
Reclassification of (income) losses from accumulated other comprehensive income, tax	(2)	(3)
Foreign Currency Translation [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	(82)	(125)
Other comprehensive income (loss) before reclassifications, after tax	3	(84)
Other comprehensive loss	3	(84)
Amounts attributable to noncontrolling interests	(1)	8
Balance	(80)	(201)
Other Comprehensive Income (Loss), Tax [Abstract]
Other comprehensive income (loss) before reclassifications, tax	0	0
Reclassification of (income) losses from accumulated other comprehensive income, tax	$ 0	$ 0